documentation update from tchrist

	Message-Id: <199806140419.WAA20549@chthon.perl.com>
	Subject: doc patches

p4raw-id: //depot/perl@1132

1 files changed, 258 insertions, 77 deletions

diff --git a/pod/perlop.pod b/pod/perlop.pod
index cae38ebf55..fe6ba1e90f 100644
--- a/pod/perlop.pod
+++ b/pod/perlop.pod
@@ -38,6 +38,8 @@ operate on scalar values only, not array values.
 
 In the following sections, these operators are covered in precedence order.
 
+Many operators can be overloaded for objects.  See L<overload>.
+
 =head1 DESCRIPTION
 
 =head2 Terms and List Operators (Leftward)
@@ -114,7 +116,7 @@ The auto-increment operator has a little extra builtin magic to it.  If
 you increment a variable that is numeric, or that has ever been used in
 a numeric context, you get a normal increment.  If, however, the
 variable has been used in only string contexts since it was set, and
-has a value that is not null and matches the pattern
+has a value that is not the empty string and matches the pattern
 C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
 character within its range, with carry:
 
@@ -144,8 +146,9 @@ starts with a plus or minus, a string starting with the opposite sign
 is returned.  One effect of these rules is that C<-bareword> is equivalent
 to C<"-bareword">.
 
-Unary "~" performs bitwise negation, i.e., 1's complement.
-(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Unary "~" performs bitwise negation, i.e., 1's complement.  For example,
+C<0666 &~ 027> is 0640.  (See also L<Integer Arithmetic> and L<Bitwise
+String Operators>.)
 
 Unary "+" has no effect whatsoever, even on strings.  It is useful
 syntactically for separating a function name from a parenthesized expression
@@ -184,16 +187,18 @@ operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
 C<$a> minus the largest multiple of C<$b> that is not greater than
 C<$a>.  If C<$b> is negative, then C<$a % $b> is C<$a> minus the
 smallest multiple of C<$b> that is not less than C<$a> (i.e. the
-result will be less than or equal to zero).
+result will be less than or equal to zero).  If C<use integer> is
+in effect, the native hardware will be used instead of this rule,
+which may be construed a bug that will be fixed at some point.
 
-Note than when C<use integer> is in scope "%" give you direct access
+Note than when C<use integer> is in scope, "%" give you direct access
 to the modulus operator as implemented by your C compiler.  This
 operator is not as well defined for negative operands, but it will
 execute faster.
 
-Binary "x" is the repetition operator.  In a scalar context, it
+Binary "x" is the repetition operator.  In scalar context, it
 returns a string consisting of the left operand repeated the number of
-times specified by the right operand.  In a list context, if the left
+times specified by the right operand.  In list context, if the left
 operand is a list in parentheses, it repeats the list.
 
     print '-' x 80;		# print row of dashes
@@ -336,11 +341,18 @@ way to find out the home directory (assuming it's not "0") might be:
     $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
 	(getpwuid($<))[7] || die "You're homeless!\n";
 
-As more readable alternatives to C<&&> and C<||>, Perl provides "and" and
-"or" operators (see below).  The short-circuit behavior is identical.  The
-precedence of "and" and "or" is much lower, however, so that you can
-safely use them after a list operator without the need for
-parentheses:
+In particular, this means that you shouldn't use this
+for selecting between two aggregates for assignment:
+
+    @a = @b || @c;		# this is wrong
+    @a = scalar(@b) || @c;	# really meant this
+    @a = @b ? @b : @c;		# this works fine, though
+
+As more readable alternatives to C<&&> and C<||> when used for
+control flow, Perl provides C<and> and C<or> operators (see below).
+The short-circuit behavior is identical.  The precedence of "and" and
+"or" is much lower, however, so that you can safely use them after a
+list operator without the need for parentheses:
 
     unlink "alpha", "beta", "gamma"
 	    or gripe(), next LINE;
@@ -350,10 +362,12 @@ With the C-style operators that would have been written like this:
     unlink("alpha", "beta", "gamma")
 	    || (gripe(), next LINE);
 
-=head2 Range Operator
+Use "or" for assignment is unlikely to do what you want; see below.
+
+=head2 Range Operators
 
 Binary ".." is the range operator, which is really two different
-operators depending on the context.  In a list context, it returns an
+operators depending on the context.  In list context, it returns an
 array of values counting (by ones) from the left value to the right
 value.  This is useful for writing C<for (1..10)> loops and for doing
 slice operations on arrays.  Be aware that under the current implementation,
@@ -364,7 +378,7 @@ write something like this:
 	# code
     }
 
-In a scalar context, ".." returns a boolean value.  The operator is
+In scalar context, ".." returns a boolean value.  The operator is
 bistable, like a flip-flop, and emulates the line-range (comma) operator
 of B<sed>, B<awk>, and various editors.  Each ".." operator maintains its
 own boolean state.  It is false as long as its left operand is false.
@@ -378,7 +392,7 @@ If you don't want it to test the right operand till the next evaluation
 operand is not evaluated while the operator is in the "false" state, and
 the left operand is not evaluated while the operator is in the "true"
 state.  The precedence is a little lower than || and &&.  The value
-returned is either the null string for false, or a sequence number
+returned is either the empty string for false, or a sequence number
 (beginning with 1) for true.  The sequence number is reset for each range
 encountered.  The final sequence number in a range has the string "E0"
 appended to it, which doesn't affect its numeric value, but gives you
@@ -394,13 +408,22 @@ As a scalar operator:
     next line if (1 .. /^$/);	# skip header lines
     s/^/> / if (/^$/ .. eof());	# quote body
 
+    # parse mail messages
+    while (<>) {
+        $in_header =   1  .. /^$/;
+        $in_body   = /^$/ .. eof();
+	# do something based on those
+    } continue {
+	close ARGV if eof; 		# reset $. each file
+    }
+
 As a list operator:
 
     for (101 .. 200) { print; }	# print $_ 100 times
     @foo = @foo[0 .. $#foo];	# an expensive no-op
     @foo = @foo[$#foo-4 .. $#foo];	# slice last 5 items
 
-The range operator (in a list context) makes use of the magical
+The range operator (in list context) makes use of the magical
 auto-increment algorithm if the operands are strings.  You
 can say
 
@@ -443,6 +466,19 @@ legal lvalues (meaning that you can assign to them):
 
 This is not necessarily guaranteed to contribute to the readability of your program.
 
+Because this operator produces an assignable result, using assignments
+without parentheses will get you in trouble.  For example, this:
+
+    $a % 2 ? $a += 10 : $a += 2
+
+Really means this:
+
+    (($a % 2) ? ($a += 10) : $a) += 2
+
+Rather than this:
+
+    ($a % 2) ? ($a += 10) : ($a += 2)
+
 =head2 Assignment Operators
 
 "=" is the ordinary assignment operator.
@@ -485,11 +521,11 @@ is equivalent to
 
 =head2 Comma Operator
 
-Binary "," is the comma operator.  In a scalar context it evaluates
+Binary "," is the comma operator.  In scalar context it evaluates
 its left argument, throws that value away, then evaluates its right
 argument and returns that value.  This is just like C's comma operator.
 
-In a list context, it's just the list argument separator, and inserts
+In list context, it's just the list argument separator, and inserts
 both its arguments into the list.
 
 The =E<gt> digraph is mostly just a synonym for the comma operator.  It's useful for
@@ -524,9 +560,27 @@ expression is evaluated only if the left expression is true.
 =head2 Logical or and Exclusive Or
 
 Binary "or" returns the logical disjunction of the two surrounding
-expressions.  It's equivalent to || except for the very low
-precedence.  This means that it short-circuits: i.e., the right
-expression is evaluated only if the left expression is false.
+expressions.  It's equivalent to || except for the very low precedence.
+This makes it useful for control flow
+
+    print FH $data		or die "Can't write to FH: $!";
+
+This means that it short-circuits: i.e., the right expression is evaluated
+only if the left expression is false.  Due to its precedence, you should
+probably avoid using this for assignment, only for control flow.
+
+    $a = $b or $c;		# bug: this is wrong
+    ($a = $b) or $c;		# really means this
+    $a = $b || $c;		# better written this way
+
+However, when it's a list context assignment and you're trying to use
+"||" for control flow, you probably need "or" so that the assignment
+takes higher precedence.
+
+    @info = stat($file) || die;     # oops, scalar sense of stat!
+    @info = stat($file) or die;     # better, now @info gets its due
+
+Then again, you could always use parentheses.
 
 Binary "xor" returns the exclusive-OR of the two surrounding expressions.
 It cannot short circuit, of course.
@@ -586,7 +640,7 @@ or "C<@>" are interpolated, as are the following sequences. Within
 a transliteration, the first ten of these sequences may be used.
 
     \t		tab             (HT, TAB)
-    \n		newline         (LF, NL)
+    \n		newline         (NL)
     \r		return          (CR)
     \f		form feed       (FF)
     \b		backspace       (BS)
@@ -606,6 +660,20 @@ a transliteration, the first ten of these sequences may be used.
 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
 and C<\U> is taken from the current locale.  See L<perllocale>.
 
+All systems use the virtual C<"\n"> to represent a line terminator,
+called a "newline".  There is no such thing as an unvarying, physical
+newline character.  It is an illusion that the operating system,
+device drivers, C libraries, and Perl all conspire to preserve.  Not all
+systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF.  For example,
+on a Mac, these are reversed, and on systems without line terminator,
+printing C<"\n"> may emit no actual data.  In general, use C<"\n"> when
+you mean a "newline" for your system, but use the literal ASCII when you
+need an exact character.  For example, most networking protocols expect
+and prefer a CR+LF (C<"\012\015"> or C<"\cJ\cM">) for line terminators,
+and although they often accept just C<"\012">, they seldom tolerate just
+C<"\015">.  If you get in the habit of using C<"\n"> for networking,
+you may be burned some day.
+
 You cannot include a literal C<$> or C<@> within a C<\Q> sequence. 
 An unescaped C<$> or C<@> interpolates the corresponding variable, 
 while escaping will cause the literal string C<\$> to be inserted.
@@ -637,6 +705,14 @@ optimization when you want to see only the first occurrence of
 something in each file of a set of files, for instance.  Only C<??>
 patterns local to the current package are reset.
 
+    while (<>) {
+	if (?^$?) {
+			    # blank line between header and body
+	}
+    } continue {
+	reset if eof;	    # clear ?? status for next file
+    }
+
 This usage is vaguely deprecated, and may be removed in some future
 version of Perl.
 
@@ -644,13 +720,13 @@ version of Perl.
 
 =item /PATTERN/cgimosx
 
-Searches a string for a pattern match, and in a scalar context returns
+Searches a string for a pattern match, and in scalar context returns
 true (1) or false ('').  If no string is specified via the C<=~> or
 C<!~> operator, the $_ string is searched.  (The string specified with
 C<=~> need not be an lvalue--it may be the result of an expression
 evaluation, but remember the C<=~> binds rather tightly.)  See also
 L<perlre>.
-See L<perllocale> for discussion of additional considerations which apply
+See L<perllocale> for discussion of additional considerations that apply
 when C<use locale> is in effect.
 
 Options are:
@@ -680,8 +756,8 @@ interpolating won't change over the life of the script.  However, mentioning
 C</o> constitutes a promise that you won't change the variables in the pattern.
 If you change them, Perl won't even notice.
 
-If the PATTERN evaluates to a null string, the last
-successfully matched regular expression is used instead.
+If the PATTERN evaluates to the empty string, the last
+I<successfully> matched regular expression is used instead.
 
 If used in a context that requires a list value, a pattern match returns a
 list consisting of the subexpressions matched by the parentheses in the
@@ -714,12 +790,12 @@ the pattern matched.
 
 The C</g> modifier specifies global pattern matching--that is, matching
 as many times as possible within the string.  How it behaves depends on
-the context.  In a list context, it returns a list of all the
+the context.  In list context, it returns a list of all the
 substrings matched by all the parentheses in the regular expression.
 If there are no parentheses, it returns a list of all the matched
 strings, as if there were parentheses around the whole pattern.
 
-In a scalar context, C<m//g> iterates through the string, returning TRUE
+In scalar context, C<m//g> iterates through the string, returning TRUE
 each time it matches, and FALSE when it eventually runs out of matches.
 (In other words, it remembers where it left off last time and restarts
 the search at that point.  You can actually find the current match
@@ -823,17 +899,53 @@ A double-quoted, interpolated string.
 
 =item `STRING`
 
-A string which is interpolated and then executed as a system command.
-The collected standard output of the command is returned.  In scalar
-context, it comes back as a single (potentially multi-line) string.
-In list context, returns a list of lines (however you've defined lines
-with $/ or $INPUT_RECORD_SEPARATOR).
+A string which is (possibly) interpolated and then executed as a system
+command with C</bin/sh> or its equivalent.  Shell wildcards, pipes,
+and redirections will be honored.  The collected standard output of the
+command is returned; standard error is unaffected.  In scalar context,
+it comes back as a single (potentially multi-line) string.  In list
+context, returns a list of lines (however you've defined lines with $/
+or $INPUT_RECORD_SEPARATOR).
+
+Because backticks do not affect standard error, use shell file descriptor
+syntax (assuming the shell supports this) if you care to address this.
+To capture a command's STDERR and STDOUT together:
 
-    $today = qx{ date };
+    $output = `cmd 2>&1`;
+
+To capture a command's STDOUT but discard its STDERR:
+
+    $output = `cmd 2>/dev/null`;
+
+To capture a command's STDERR but discard its STDOUT (ordering is
+important here):
+
+    $output = `cmd 2>&1 1>/dev/null`;
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out the old STDERR:
+
+    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+and safest to redirect them separately to files, and then read from those
+files when the program is done:
+
+    system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+
+Using single-quote as a delimiter protects the command from Perl's
+double-quote interpolation, passing it on to the shell instead:
+
+    $perl_info  = qx(ps $$);            # that's Perl's $$
+    $shell_info = qx'ps $$';            # that's the new shell's $$
+
+Note that how the string gets evaluated is entirely subject to the command
+interpreter on your system.  On most platforms, you will have to protect
+shell metacharacters if you want them treated literally.  This is in
+practice difficult to do, as it's unclear how to escape which characters.
+See L<perlsec> for a clean and safe example of a manual fork() and exec()
+to emulate backticks safely.
 
-Note that how the string gets evaluated is entirely subject to the
-command interpreter on your system.  On most platforms, you will have
-to protect shell metacharacters if you want them treated literally.
 On some platforms (notably DOS-like ones), the shell may not be
 capable of dealing with multiline commands, so putting newlines in
 the string may not get you what you want.  You may be able to evaluate
@@ -846,8 +958,14 @@ of the command line.  You must ensure your strings don't exceed this
 limit after any necessary interpolations.  See the platform-specific
 release notes for more details about your particular environment.
 
-Also realize that using this operator frequently leads to unportable
-programs.
+Using this operator can lead to programs that are difficult to port,
+because the shell commands called vary between systems, and may in
+fact not be present at all.  As one example, the C<type> command under
+the POSIX shell is very different from the C<type> command under DOS.
+That doesn't mean you should go out of your way to avoid backticks
+when they're the right way to get something done.  Perl was made to be
+a glue language, and one of the things it glues together is commands.
+Just understand what you're getting yourself into.
 
 See L<"I/O Operators"> for more discussion.
 
@@ -858,13 +976,16 @@ whitespace as the word delimiters.  It is exactly equivalent to
 
     split(' ', q/STRING/);
 
+This equivalency means that if used in scalar context, you'll get split's
+(unfortunate) scalar context behavior, complete with mysterious warnings.
+
 Some frequently seen examples:
 
     use POSIX qw( setlocale localeconv )
     @EXPORT = qw( foo bar baz );
 
 A common mistake is to try to separate the words with comma or to put
-comments into a multi-line qw-string.  For this reason the C<-w>
+comments into a multi-line C<qw>-string.  For this reason the C<-w>
 switch produce warnings if the STRING contains the "," or the "#"
 character.
 
@@ -876,7 +997,7 @@ made.  Otherwise it returns false (specifically, the empty string).
 
 If no string is specified via the C<=~> or C<!~> operator, the C<$_>
 variable is searched and modified.  (The string specified with C<=~> must
-be a scalar variable, an array element, a hash element, or an assignment
+be scalar variable, an array element, a hash element, or an assignment
 to one of those, i.e., an lvalue.)
 
 If the delimiter chosen is single quote, no variable interpolation is
@@ -885,9 +1006,9 @@ PATTERN contains a $ that looks like a variable rather than an
 end-of-string test, the variable will be interpolated into the pattern
 at run-time.  If you want the pattern compiled only once the first time
 the variable is interpolated, use the C</o> option.  If the pattern
-evaluates to a null string, the last successfully executed regular
+evaluates to the empty string, the last successfully executed regular
 expression is used instead.  See L<perlre> for further explanation on these.
-See L<perllocale> for discussion of additional considerations which apply
+See L<perllocale> for discussion of additional considerations that apply
 when C<use locale> is in effect.
 
 Options are:
@@ -920,9 +1041,9 @@ Examples:
 
     s/Login: $foo/Login: $bar/; # run-time pattern
 
-    ($foo = $bar) =~ s/this/that/;
+    ($foo = $bar) =~ s/this/that/;	# copy first, then change
 
-    $count = ($paragraph =~ s/Mister\b/Mr./g);
+    $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count
 
     $_ = 'abc123xyz';
     s/\d+/$&*2/e;		# yields 'abc246xyz'
@@ -933,18 +1054,27 @@ Examples:
     s/%(.)/$percent{$1} || $&/ge;	# expr now, so /e
     s/^=(\w+)/&pod($1)/ge;	# use function call
 
+    # expand variables in $_, but dynamics only, using
+    # symbolic dereferencing
+    s/\$(\w+)/${$1}/g;
+
     # /e's can even nest;  this will expand
-    # simple embedded variables in $_
+    # any embedded scalar variable (including lexicals) in $_
     s/(\$\w+)/$1/eeg;
 
-    # Delete C comments.
+    # Delete (most) C comments.
     $program =~ s {
 	/\*	# Match the opening delimiter.
 	.*?	# Match a minimal number of characters.
 	\*/	# Match the closing delimiter.
     } []gsx;
 
-    s/^\s*(.*?)\s*$/$1/;	# trim white space
+    s/^\s*(.*?)\s*$/$1/;	# trim white space in $_, expensively
+
+    for ($variable) {		# trim white space in $variable, cheap
+	s/^\s+//;
+	s/\s+$//;
+    }
 
     s/([^ ]*) *([^ ]*)/$2 $1/;	# reverse 1st two fields
 
@@ -998,7 +1128,7 @@ character.
 If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
 exactly as specified.  Otherwise, if the REPLACEMENTLIST is shorter
 than the SEARCHLIST, the final character is replicated till it is long
-enough.  If the REPLACEMENTLIST is null, the SEARCHLIST is replicated.
+enough.  If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
 This latter is useful for counting characters in a class or for
 squashing character sequences in a class.
 
@@ -1045,8 +1175,8 @@ There are several I/O operators you should know about.
 A string enclosed by backticks (grave accents) first undergoes
 variable substitution just like a double quoted string.  It is then
 interpreted as a command, and the output of that command is the value
-of the pseudo-literal, like in a shell.  In a scalar context, a single
-string consisting of all the output is returned.  In a list context,
+of the pseudo-literal, like in a shell.  In scalar context, a single
+string consisting of all the output is returned.  In list context,
 a list of values is returned, one for each line of output.  (You can
 set C<$/> to use a different line terminator.)  The command is executed
 each time the pseudo-literal is evaluated.  The status value of the
@@ -1066,7 +1196,7 @@ situation where an automatic assignment happens.  I<If and ONLY if> the
 input symbol is the only thing inside the conditional of a C<while> or
 C<for(;;)> loop, the value is automatically assigned to the variable
 C<$_>.  In these loop constructs, the assigned value (whether assignment
-is automatic or explcit) is then tested to see if it is defined.
+is automatic or explicit) is then tested to see if it is defined.
 The defined test avoids problems where line has a string value
 that would be treated as false by perl e.g. "" or "0" with no trailing
 newline. (This may seem like an odd thing to you, but you'll use the 
@@ -1086,12 +1216,12 @@ and this also behaves similarly, but avoids the use of $_ :
     while (my $line = <STDIN>) { print $line }    
 
 If you really mean such values to terminate the loop they should be 
-tested for explcitly:
+tested for explicitly:
 
     while (($_ = <STDIN>) ne '0') { ... }
     while (<STDIN>) { last unless $_; ... }
 
-In other boolean contexts C<E<lt>I<filehandle>E<gt>> without explcit C<defined>
+In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
 test or comparison will solicit a warning if C<-w> is in effect.
 
 The filehandles STDIN, STDOUT, and STDERR are predefined.  (The
@@ -1109,7 +1239,7 @@ The null filehandle E<lt>E<gt> is special and can be used to emulate the
 behavior of B<sed> and B<awk>.  Input from E<lt>E<gt> comes either from
 standard input, or from each file listed on the command line.  Here's
 how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
-checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened
+checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
 gives you standard input.  The @ARGV array is then processed as a list
 of filenames.  The loop
 
@@ -1136,10 +1266,19 @@ doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
 You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
 containing the list of filenames you really want.  Line numbers (C<$.>)
 continue as if the input were one big happy file.  (But see example
-under eof() for how to reset line numbers on each file.)
+under C<eof> for how to reset line numbers on each file.)
+
+If you want to set @ARGV to your own list of files, go right ahead.  
+This sets @ARGV to all plain text files if no @ARGV was given:
+
+    @ARGV = grep { -f && -T } glob('*') unless @ARGV;
 
-If you want to set @ARGV to your own list of files, go right ahead.  If
-you want to pass switches into your script, you can use one of the
+You can even set them to pipe commands.  For example, this automatically
+filters compressed arguments through B<gzip>:
+
+    @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
+
+If you want to pass switches into your script, you can use one of the
 Getopts modules or put a loop on the front like this:
 
     while ($_ = $ARGV[0], /^-/) {
@@ -1147,10 +1286,11 @@ Getopts modules or put a loop on the front like this:
         last if /^--$/;
 	if (/^-D(.*)/) { $debug = $1 }
 	if (/^-v/)     { $verbose++  }
-	...		# other switches
+	# ...		# other switches
     }
+
     while (<>) {
-	...		# code for each line
+	# ...		# code for each line
     }
 
 The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.  
@@ -1159,22 +1299,28 @@ If you call it again after this it will assume you are processing another
 
 If the string inside the angle brackets is a reference to a scalar
 variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the
-filehandle to input from, or a reference to the same.  For example:
+filehandle to input from, or its typeglob, or a reference to the same.  For example:
 
     $fh = \*STDIN;
     $line = <$fh>;
 
-If the string inside angle brackets is not a filehandle or a scalar
-variable containing a filehandle name or reference, then it is interpreted
-as a filename pattern to be globbed, and either a list of filenames or the
-next filename in the list is returned, depending on context.  One level of
-$ interpretation is done first, but you can't say C<E<lt>$fooE<gt>>
-because that's an indirect filehandle as explained in the previous
-paragraph.  (In older versions of Perl, programmers would insert curly
-brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>.
-These days, it's considered cleaner to call the internal function directly
-as C<glob($foo)>, which is probably the right way to have done it in the
-first place.)  Example:
+If what's within the angle brackets is neither a filehandle nor a simple
+scalar variable containing a filehandle name, typeglob, or typeglob
+reference, it is interpreted as a filename pattern to be globbed, and
+either a list of filenames or the next filename in the list is returned,
+depending on context.   This distinction is determined on syntactic
+grounds alone.  That means C<E<lt>$xE<gt>> is always a readline from
+an indirect handle, but C<E<lt>$hash{key}E<gt>> is always a glob.
+That's because $x is a simple scalar variable, but C<$hash{key}> is
+not--it's a hash element.
+
+One level of double-quote interpretation is done first, but you can't
+say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
+in the previous paragraph.  (In older versions of Perl, programmers
+would insert curly brackets to force interpretation as a filename glob:
+C<E<lt>${foo}E<gt>>.  These days, it's considered cleaner to call the
+internal function directly as C<glob($foo)>, which is probably the right
+way to have done it in the first place.)  Example:
 
     while (<*.c>) {
 	chmod 0644, $_;
@@ -1202,7 +1348,7 @@ long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
 A glob evaluates its (embedded) argument only when it is starting a new
 list.  All values must be read before it will start over.  In a list
 context this isn't important, because you automatically get them all
-anyway.  In a scalar context, however, the operator returns the next value
+anyway.  In scalar context, however, the operator returns the next value
 each time it is called, or a C<undef> value if you've just run out. As
 for filehandles an automatic C<defined> is generated when the glob
 occurs in the test part of a C<while> or C<for> - because legal glob returns
@@ -1229,7 +1375,7 @@ to become confused with the indirect filehandle notation.
 =head2 Constant Folding
 
 Like C, Perl does a certain amount of expression evaluation at
-compile time, whenever it determines that all of the arguments to an
+compile time, whenever it determines that all arguments to an
 operator are static and have no side effects.  In particular, string
 concatenation happens at compile time between literals that don't do
 variable substitution.  Backslash interpretation also happens at
@@ -1242,7 +1388,7 @@ and this all reduces to one string internally.  Likewise, if
 you say
 
     foreach $file (@filenames) {
-	if (-s $file > 5 + 100 * 2**16) { ... }
+	if (-s $file > 5 + 100 * 2**16) {  }
     }
 
 the compiler will precompute the number that
@@ -1299,7 +1445,7 @@ However, C<use integer> still has meaning
 for them.  By default, their results are interpreted as unsigned
 integers.  However, if C<use integer> is in effect, their results are
 interpreted as signed integers.  For example, C<~0> usually evaluates
-to a large integral value.  However, C<use integer; ~0> is -1.
+to a large integral value.  However, C<use integer; ~0> is -1 on twos-complement machines.
 
 =head2 Floating-point Arithmetic
 
@@ -1308,6 +1454,27 @@ similar ways to provide rounding or truncation at a certain number of
 decimal places.  For rounding to a certain number of digits, sprintf()
 or printf() is usually the easiest route.
 
+Floating-point numbers are only approximations to what a mathematician
+would call real numbers.  There are infinitely more reals than floats,
+so some corners must be cut.  For example:
+
+    printf "%.20g\n", 123456789123456789;
+    #        produces 123456789123456784
+
+Testing for exact equality of floating-point equality or inequality is
+not a good idea.  Here's a (relatively expensive) work-around to compare
+whether two floating-point numbers are equal to a particular number of
+decimal places.  See Knuth, volume II, for a more robust treatment of
+this topic.
+
+    sub fp_equal {
+	my ($X, $Y, $POINTS) = @_;
+	my ($tX, $tY);
+	$tX = sprintf("%.${POINTS}g", $X);
+	$tY = sprintf("%.${POINTS}g", $Y);
+	return $tX eq $tY;
+    }
+
 The POSIX module (part of the standard perl distribution) implements
 ceil(), floor(), and a number of other mathematical and trigonometric
 functions.  The Math::Complex module (part of the standard perl
@@ -1320,3 +1487,17 @@ the rounding method used should be specified precisely.  In these
 cases, it probably pays not to trust whichever system rounding is
 being used by Perl, but to instead implement the rounding function you
 need yourself.
+
+=head2 Bigger Numbers
+
+The standard Math::BigInt and Math::BigFloat modules provide
+variable precision arithmetic and overloaded operators.
+At the cost of some space and considerable speed, they
+avoid the normal pitfalls associated with limited-precision
+representations.
+
+    use Math::BigInt;
+    $x = Math::BigInt->new('123456789123456789');
+    print $x * $x;
+
+    # prints +15241578780673678515622620750190521