perl 5.003_01: pod/perltrap.pod

Typos and pod formatting corrected
Perl4 - Perl5 traps revised extensively

1 files changed, 936 insertions, 104 deletions

diff --git a/pod/perltrap.pod b/pod/perltrap.pod
index dd219c064b..be58bc52e2 100644
--- a/pod/perltrap.pod
+++ b/pod/perltrap.pod
@@ -58,7 +58,7 @@ comparisons.
 =item *
 
 Reading an input line does not split it for you.  You get to split it
-yourself to an array.  And split() operator has different
+yourself to an array.  And the split() operator has different
 arguments.
 
 =item *
@@ -172,7 +172,7 @@ Variables begin with "$" or "@" in Perl.
 
 =item *
 
-printf() does not implement the "*" format for interpolating
+C<printf()> does not implement the "*" format for interpolating
 field widths, but it's trivial to use interpolation of double-quoted
 strings to achieve the same effect.
 
@@ -231,7 +231,7 @@ Sharp shell programmers should take note of the following:
 
 =item *
 
-The backtick operator does variable interpretation without regard to
+The backtick operator does variable interpolation without regard to
 the presence of single quotes in the command.
 
 =item *
@@ -293,7 +293,7 @@ People have a hard time remembering that some functions
 default to $_, or @ARGV, or whatever, but that others which
 you might expect to do not.  
 
-=item * 
+=item *
 
 The <FH> construct is not the name of the filehandle, it is a readline
 operation on that handle.  The data read is only assigned to $_ if the
@@ -303,7 +303,7 @@ file read is the sole condition in a while loop:
     while ($_ = <FH>) { }..
     <FH>;  # data discarded!
 
-=item * 
+=item *
 
 Remember not to use "C<=>" when you need "C<=~>";
 these two constructs are quite different:
@@ -318,9 +318,9 @@ loop control on.
 
 =item *
 
-Use my() for local variables whenever you can get away with 
+Use C<my()> for local variables whenever you can get away with 
 it (but see L<perlform> for where you can't).  
-Using local() actually gives a local value to a global 
+Using C<local()> actually gives a local value to a global 
 variable, which leaves you open to unforeseen side-effects
 of dynamic scoping.
 
@@ -332,75 +332,567 @@ external name is still an alias for the original.
 
 =back
 
-=head2 Perl4 Traps
+=head2 Perl4 to Perl5  Traps
 
-Penitent Perl 4 Programmers should take note of the following
-incompatible changes that occurred between release 4 and release 5:
+Practicing Perl4 Programmers should take note of the following 
+Perl4-to-Perl5 specific traps.
+
+They're crudely ordered according to the following list:
 
 =over 4
 
-=item *
+=item Discontinuance, Deprecation, and BugFix traps
 
-C<@> now always interpolates an array in double-quotish strings.  Some programs
-may now need to use backslash to protect any C<@> that shouldn't interpolate.
+Anything that's been fixed as a perl4 bug, removed as a perl4 feature
+or deprecated as a perl4 feature with the intent to encourage usage of
+some other perl5 feature.
 
-=item *
+=item Parsing Traps
 
-Barewords that used to look like strings to Perl will now look like subroutine
-calls if a subroutine by that name is defined before the compiler sees them.
-For example:
+Traps that appear to stem from the new parser.
 
-    sub SeeYa { die "Hasta la vista, baby!" }
-    $SIG{'QUIT'} = SeeYa;
+=item Numerical Traps
 
-In Perl 4, that set the signal handler; in Perl 5, it actually calls the
-function!  You may use the B<-w> switch to find such places.
+Traps having to do with numerical or mathematical operators.
 
-=item *
+=item General data type traps
 
-Symbols starting with C<_> are no longer forced into package C<main>, except
-for $_ itself (and @_, etc.).
+Traps involving perl standard data types.
 
-=item *
+=item Context Traps - scalar, list contexts
+
+Traps related to context within lists, scalar statements/declarations.
+
+=item Precedence Traps
+
+Traps related to the precedence of parsing, evaluation, and execution of
+code.
+
+=item General Regular Expression Traps using s///, etc.
+
+Traps related to the use of pattern matching.
+
+=item Subroutine, Signal, Sorting Traps
+
+Traps related to the use of signals and signal handlers, general subroutines,
+and sorting, along with sorting subroutines.
+
+=item OS Traps
+
+OS-specific traps.
+
+=item DBM Traps
+
+Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
+
+=item Unclassified Traps
+
+Everything else.
+
+=back
+
+If you find an example of a conversion trap that is not listed here,
+please submit it to Bill Middleton F<wjm@best.com> for inclusion.
+Also note that at least some of these can be caught with C<-w>.
+
+=head2 Discontinuance, Deprecation, and BugFix traps
+
+Anything that has been discontinued, deprecated, or fixed as
+a bug from perl4.  
 
-Double-colon is now a valid package separator in an identifier.  Thus these
-behave differently in perl4 vs. perl5:
+=over 4
+
+=item * Discontinuance 
+
+Symbols starting with "_" are no longer forced into package main, except
+for C<$_> itself (and C<@_>, etc.).
+
+    package test;
+    $_legacy = 1;
 
-    print "$a::$b::$c\n";
+    package main;
+    print "\$_legacy is ",$_legacy,"\n";
+ 
+    # perl4 prints: $_legacy is 1
+    # perl5 prints: $_legacy is
+
+=item * Deprecation 
+
+Double-colon is now a valid package separator in a variable name.  Thus these
+behave differently in perl4 vs. perl5, since the packages don't exist.
+
+    $a=1;$b=2;$c=3;$var=4;
+    print "$a::$b::$c ";
     print "$var::abc::xyz\n";
+ 
+    # perl4 prints: 1::2::3 4::abc::xyz
+    # perl5 prints: 3
 
-=item *
+Given that C<::> is now the preferred package delimiter, it is debatable
+whether this should be classed as a bug or not.
+(The older package delimiter, ' ,is used here)
 
-C<s'$lhs'$rhs'> now does no interpolation on either side.  It used to
-interpolate C<$lhs> but not C<$rhs>.
+    $x = 10 ;
+    print "x=${'x}\n" ;
+ 
+    # perl4 prints: x=10
+    # perl5 prints: Can't find string terminator "'" anywhere before EOF
 
-=item *
+Also see precedence traps, for parsing C<$:>. 
 
-The second and third arguments of splice() are now evaluated in scalar
-context (as the book says) rather than list context.
+=item * BugFix
 
-=item *
+The second and third arguments of C<splice()> are now evaluated in scalar
+context (as the Camel says) rather than list context.
 
-These are now semantic errors because of precedence:
+    sub sub1{return(0,2) }          # return a 2-elem array
+    sub sub2{ return(1,2,3)}        # return a 3-elem array
+    @a1 = ("a","b","c","d","e"); 
+    @a2 = splice(@a1,&sub1,&sub2);
+    print join(' ',@a2),"\n";
+ 
+    # perl4 prints: a b
+    # perl5 prints: c d e 
 
-    shift @list + 20;	
-    $n = keys %map + 20; 
+=item * Discontinuance 
 
-Because if that were to work, then this couldn't:
+You can't do a C<goto> into a block that is optimized away.  Darn.
 
-    sleep $dormancy + 20;
+    goto marker1;
 
-=item *
+    for(1){         
+    marker1:
+        print "Here I is!\n";
+    } 
+ 
+    # perl4 prints: Here I is!
+    # perl5 dumps core (SEGV)
+
+=item * Discontinuance 
+
+It is no longer syntactically legal to use whitespace as the name
+of a variable, or as a delimiter for any kind of quote construct.
+Double darn. 
+
+    $a = ("foo bar");
+    $b = q baz ;
+    print "a is $a, b is $b\n";
+ 
+    # perl4 prints: a is foo bar, b is baz
+    # perl5 errors: Bare word found where operator expected
+    
+=item * Discontinuance
+
+The archaic while/if BLOCK BLOCK syntax is no longer supported.
+
+    if { 1 } {
+        print "True!";
+    }
+    else {
+        print "False!";
+    }
+ 
+    # perl4 prints: True!
+    # perl5 errors: syntax error at test.pl line 1, near "if {"
+
+=item * BugFix
+
+The C<**> operator now binds more tightly than unary minus.
+It was documented to work this way before, but didn't.
+
+    print -4**2,"\n";
+ 
+    # perl4 prints: 16
+    # perl5 prints: -16
+
+=item * Discontinuance 
+
+The meaning of C<foreach{}> has changed slightly when it is iterating over a
+list which is not an array.  This used to assign the list to a
+temporary array, but no longer does so (for efficiency).  This means
+that you'll now be iterating over the actual values, not over copies of
+the values.  Modifications to the loop variable can change the original
+values.
+
+    @list = ('ab','abc','bcd','def');
+    foreach $var (grep(/ab/,@list)){
+        $var = 1;
+    }
+    print (join(':',@list));
+ 
+    # perl4 prints: ab:abc:bcd:def
+    # perl5 prints: 1:1:bcd:def
+
+To retain Perl4 semantics you need to assign your list
+explicitly to a temporary array and then iterate over that.  For 
+example, you might need to change
+
+    foreach $var (grep(/ab/,@list)){
+
+to
+
+    foreach $var (@tmp = grep(/ab/,@list)){
+
+Otherwise changing $var will clobber the values of @list.  (This most often
+happens when you use C<$_> for the loop variable, and call subroutines in
+the loop that don't properly localize C<$_>.)
+
+=item * Deprecation
+
+Some error messages will be different.
+
+=item * Discontinuance 
+
+Some bugs may have been inadvertently removed.  :-)
+
+=back
+
+=head2 Parsing Traps
+
+Perl4-to-Perl5 traps from having to do with parsing.
+
+=over 4
+
+=item * Parsing
+
+Note the space between . and =
+
+    $string . = "more string";
+    print $string;
+ 
+    # perl4 prints: more string
+    # perl5 prints: syntax error at - line 1, near ". ="
+
+=item * Parsing
+
+Better parsing in perl 5
+
+    sub foo {}
+    &foo
+    print("hello, world\n");
+ 
+    # perl4 prints: hello, world
+    # perl5 prints: syntax error
+
+=item * Parsing
+
+"if it looks like a function, it is a function" rule.
+
+  print
+    ($foo == 1) ? "is one\n" : "is zero\n";
+ 
+    # perl4 prints: is zero
+    # perl5 warns: "Useless use of a constant in void context" if using -w
+
+=back
+
+=head2 Numerical Traps
+
+Perl4-to-Perl5 traps having to do with numerical operators,
+operands, or output from same.
+
+=over 5
+
+=item * Numerical
+
+Formatted output and significant digits
+
+    print 7.373504 - 0, "\n";
+    printf "%20.18f\n", 7.373504 - 0; 
+ 
+    # Perl4 prints:
+    7.375039999999996141
+    7.37503999999999614
+ 
+    # Perl5 prints:
+    7.373504
+    7.37503999999999614
+
+=item * Numerical
+
+Large integer trap with autoincrement
+
+    $a = $b = 2147483647;
+    print "$a   $b\n";
+    $a += 1;
+    $b++;
+    print "$a   $b\n";
+ 
+    # perl4 prints:
+    2147483647   2147483647
+    2147483648   2147483648
+ 
+    # perl5 prints:
+    2147483647   2147483647
+    2147483648   -2147483648
+
+=item * Numerical  
+
+Assignment of return values from numeric equality tests
+does not work in perl5 when the test evaluates to false (0).
+Logical tests now return an null, instead of 0
+ 
+    $p = ($test == 1);
+    print $p,"\n";
+  
+    # perl4 prints: 0
+    # perl5 prints:
+
+Also see the L<General Regular Expression Traps> tests for another example 
+of this new feature...
+
+=back
+
+=head2 General data type traps
+
+Perl4-to-Perl5 traps involving most data-types, and their usage
+within certain expressions and/or context.
+
+=over 5
+
+=item * (Arrays)
+
+Negative array subscripts now count from the end of the array.
+
+    @a = (1, 2, 3, 4, 5);
+    print "The third element of the array is $a[3] also expressed as $a[-2] \n";
+  
+    # perl4 prints: The third element of the array is 4 also expressed as
+    # perl5 prints: The third element of the array is 4 also expressed as 4
+
+=item * (Arrays)
+
+Setting C<$#array> lower now discards array elements, and makes them
+impossible to recover.
+
+    @a = (a,b,c,d,e); 
+    print "Before: ",join('',@a);
+    $#a =1; 
+    print ", After: ",join('',@a);
+    $#a =3;
+    print ", Recovered: ",join('',@a),"\n";
+  
+    # perl4 prints: Before: abcde, After: ab, Recovered: abcd
+    # perl5 prints: Before: abcde, After: ab, Recovered: ab
+
+=item * (Hashes)
+
+Hashes get defined before use
+
+    local($s,@a,%h); 
+    die "scalar \$s defined" if defined($s);
+    die "array \@a defined" if defined(@a);
+    die "hash \%h defined" if defined(%h);
+ 
+    # perl4 prints:
+    # perl5 dies: hash %h defined
+
+=item * (Globs)
+
+glob assignment from variable to variable will fail if the assigned
+variable is localized subsequent to the assignment
+
+    @a = ("This is Perl 4");
+    *b = *a;
+    local(@a);
+    print @b,"\n";
+ 
+    # perl4 prints: This is Perl 4
+    # perl5 prints:
+ 
+    # Another example
+ 
+    *fred = *barney; # fred is aliased to barney
+    @barney = (1, 2, 4);
+    # @fred;
+    print "@fred";  # should print "1, 2, 4"
+ 
+    # perl4 prints: 1 2 4
+    # perl5 prints: Literal @fred now requires backslash 
+ 
+=item * (Scalar String)
+
+Changes in unary negation (of strings)
+This change effects both the return value and what it
+does to auto(magic)increment.
+
+    $x = "aaa";
+    print ++$x," : ";
+    print -$x," : ";
+    print ++$x,"\n";
+ 
+    # perl4 prints: aab : -0 : 1
+    # perl5 prints: aab : -aab : aac
+
+=item * (Constants)
+
+perl 4 lets you modify constants:
+
+    $foo = "x";
+    &mod($foo);
+    for ($x = 0; $x < 3; $x++) {
+        &mod("a");
+    }
+    sub mod {
+        print "before: $_[0]";
+        $_[0] = "m";
+        print "  after: $_[0]\n";
+    }
+ 
+    # perl4:
+    # before: x  after: m
+    # before: a  after: m
+    # before: m  after: m
+    # before: m  after: m
+ 
+    # Perl5:
+    # before: x  after: m
+    # Modification of a read-only value attempted at foo.pl line 12.
+    # before: a
+
+=item * (Scalars)
+
+The behavior is slightly different for:
+
+    print "$x", defined $x
+  
+    # perl 4: 1
+    # perl 5: <no output, $x is not called into existence>
+
+=item * (Variable Suicide)
+
+Variable suicide behavior is more consistent under Perl 5.
+Perl5 exhibits the same behavior for associative arrays and scalars,
+that perl4 exhibits only for scalars.
+
+    $aGlobal{ "aKey" } = "global value";
+    print "MAIN:", $aGlobal{"aKey"}, "\n";
+    $GlobalLevel = 0;
+    &test( *aGlobal );
+
+    sub test {
+        local( *theArgument ) = @_;
+        local( %aNewLocal ); # perl 4 != 5.001l,m
+        $aNewLocal{"aKey"} = "this should never appear";  
+        print "SUB: ", $theArgument{"aKey"}, "\n";
+        $aNewLocal{"aKey"} = "level $GlobalLevel";   # what should print
+        $GlobalLevel++;
+        if( $GlobalLevel<4 ) {
+            &test( *aNewLocal );
+        }
+    }
+ 
+    # Perl4:
+    # MAIN:global value
+    # SUB: global value
+    # SUB: level 0
+    # SUB: level 1
+    # SUB: level 2
+ 
+    # Perl5:
+    # MAIN:global value
+    # SUB: global value
+    # SUB: this should never appear
+    # SUB: this should never appear
+    # SUB: this should never appear
+
+=back 
+
+=head2 Context Traps - scalar, list contexts
+
+=over 5
+
+=item * (list context)
+
+The elements of argument lists for formats are now evaluated in list
+context.  This means you can interpolate list values now.
+
+    @fmt = ("foo","bar","baz");
+    format STDOUT=
+    @<<<<< @||||| @>>>>>
+    @fmt;
+    .
+    write;  
+ 
+    # perl4 errors:  Please use commas to separate fields in file
+    # perl5 prints: foo     bar      baz
+
+=item * (scalar context)
+
+The C<caller()> function now returns a false value in a scalar context 
+if there is no caller.  This lets library files determine if they're 
+being required.
+
+    caller() ? (print "You rang?\n") : (print "Got a 0\n");
+  
+    # perl4 errors: There is no caller
+    # perl5 prints: Got a 0
+ 
+=item * (scalar context)
+
+The comma operator in a scalar context is now guaranteed to give a
+scalar context to its arguments.
+
+    @y= ('a','b','c');
+    $x = (1, 2, @y);
+    print "x = $x\n";
+ 
+    # Perl4 prints:  x = c   # Thinks list context interpolates list
+    # Perl5 prints:  x = 3   # Knows scalar uses length of list
+
+=item * (list, builtin)
+
+C<sprintf()> funkiness (array argument converted to scalar array count)
+This test could be added to t/op/sprintf.t
+
+    @z = ('%s%s', 'foo', 'bar');
+    $x = sprintf(@z);
+    if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
+ 
+    # perl4 prints: ok 2
+    # perl5 prints: not ok 2
+
+C<printf()> works fine, though:
+
+    printf STDOUT (@z);
+    print "\n"; 
+ 
+    # perl4 prints: foobar
+    # perl5 prints: foobar
+
+Probably a bug.
+
+=back
+
+=head2 Precedence Traps
+
+Perl4-to-Perl5 traps involving precedence order.
+
+=item * 
+
+These are now semantic errors because of precedence:
+
+    @list = (1,2,3,4,5);
+    %map = ("a",1,"b",2,"c",3,"d",4);
+    $n = shift @list + 2;   # first item in list plus 2
+    print "n is $n, ";
+    $m = keys %map + 2;     # number of items in hash plus 2
+    print "m is $m\n";
+ 
+    # perl4 prints: n is 3, m is 6
+    # perl5 errors and fails to compile
+
+=item * Precedence
 
 The precedence of assignment operators is now the same as the precedence
 of assignment.  Perl 4 mistakenly gave them the precedence of the associated
 operator.  So you now must parenthesize them in expressions like
 
     /foo/ ? ($a += 2) : ($a -= 2);
-
+    
 Otherwise
 
-    /foo/ ? $a += 2 : $a -= 2;
+    /foo/ ? $a += 2 : $a -= 2
 
 would be erroneously parsed as
 
@@ -408,115 +900,455 @@ would be erroneously parsed as
 
 On the other hand,
 
-    $a += /foo/ ? 1 : 2;
+    $a += /foo/ ? 1 : 2; 
 
 now works as a C programmer would expect.
 
-=item *
+=item * Precedence
 
-C<open FOO || die> is now incorrect.  You need parens around the filehandle.
-While temporarily supported, using such a construct will 
-generate a non-fatal (but non-suppressible) warning.
+    open FOO || die;
 
-=item *
+is now incorrect.  You need parens around the filehandle.
+Otherwise, perl5 leaves the statement as it's default precedence:
 
-The elements of argument lists for formats are now evaluated in list
-context.  This means you can interpolate list values now.
+    open(FOO || die);
+ 
+    # perl4 opens or dies
+    # perl5 errors: Precedence problem: open FOO should be open(FOO)
 
-=item *
+=item * Precedence
 
-You can't do a C<goto> into a block that is optimized away.  Darn.
+perl4 gives the special variable, C<$:> precedence, where perl5
+treats C<$::> as main C<package>
 
-=item *
+    $a = "x"; print "$::a";
+     
+    # perl 4 prints: -:a
+    # perl 5 prints: x
+    
+=item * Precedence
 
-It is no longer syntactically legal to use whitespace as the name
-of a variable, or as a delimiter for any kind of quote construct.
-Double darn.
+concatenation precedence over filetest operator?  
 
-=item *
+    -e $foo .= "q" 
+  
+    # perl4 prints: no output
+    # perl5 prints: Can't modify -e in concatenation
 
-The caller() function now returns a false value in a scalar context if there
-is no caller.  This lets library files determine if they're being required.
+=item * Precedence
 
-=item *
+Assignment to value takes precedence over assignment to key in
+perl5 when using the shift operator on both sides.
+
+    @arr = ( 'left', 'right' );
+    $a{shift @arr} = shift @arr;
+    print join( ' ', keys %a );
+
+    # perl4 prints: left
+    # perl5 prints: right
+
+=back
+
+=head2 General Regular Expression Traps using s///, etc.
+
+All types of RE traps.
+
+=over 5
+
+=item * Regular Expression
+
+C<s'$lhs'$rhs'> now does no interpolation on either side.  It used to
+interpolate C<$lhs> but not C<$rhs>.  (And still does not match a literal 
+'$' in string)
+
+    $a=1;$b=2;
+    $string = '1 2 $a $b';
+    $string =~ s'$a'$b';
+    print $string,"\n";
+ 
+    # perl4 prints: $b 2 $a $b
+    # perl5 prints: 1 2 $a $b
+
+=item * Regular Expression
 
 C<m//g> now attaches its state to the searched string rather than the
-regular expression.
+regular expression.  (Once the scope of a block is left for the sub, the
+state of the searched string is lost)
+
+    $_ = "ababab";
+    while(m/ab/g){
+        &doit("blah");
+    }
+    sub doit{local($_) = shift; print "Got $_ "}
+ 
+    # perl4 prints: blah blah blah
+    # perl5 prints: infinite loop blah...
+
+=item * Regular Expression
+
+If no parentheses are used in a match, Perl4 sets C<$+> to
+the whole match, just like C<$&>. Perl5 does not.
+
+    "abcdef" =~ /b.*e/;
+    print "\$+ = $+\n";
+ 
+    # perl4 prints: bcde
+    # perl5 prints:
+
+=item * Regular Expression
+
+substitution now returns the null string if it fails
+
+    $string = "test";
+    $value = ($string =~ s/foo//);
+    print $value, "\n";
+ 
+    # perl4 prints: 0
+    # perl5 prints:
+
+Also see L<Numerical Traps> for another example of this new feature.
+
+=item * Regular Expression
+
+C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no 
+backtick expansion
+
+    $string = "";
+    $string =~ s`^`hostname`;
+    print $string, "\n";
+ 
+    # perl4 prints: <the local hostname>
+    # perl5 prints: hostname
+
+=item * Regular Expression
+
+Stricter parsing of variables used in regular expressions
+
+    s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
+ 
+    # perl4: compiles w/o error
+    # perl5: with Scalar found where operator expected ..., near "$opt$plus"
+
+an added component of this example, apparently from the same script, is
+the actual value of the s'd string after the substitution.
+C<[$opt]> is a character class in perl4 and an array subscript in perl5
+
+    $grpc = 'a'; 
+    $opt  = 'r';
+    $_ = 'bar';
+    s/^([^$grpc]*$grpc[$opt]?)/foo/;
+    print ;
+ 
+    # perl4 prints: foo
+    # perl5 prints: foobar
+
+=item * Regular Expression
+
+Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
+repeatedly, like C</x/> or C<m!x!>.
+
+    $test = "once";
+    sub match { $test =~ m?once?; }
+    &match();
+    if( &match() ) {
+        # m?x? matches more then once
+        print "perl4\n";
+    } else { 
+        # m?x? matches only once
+        print "perl5\n"; 
+    }
+ 
+    # perl4 prints: perl4
+    # perl5 prints: perl5
 
-=item *
 
-C<reverse> is no longer allowed as the name of a sort subroutine.
+=back
+
+=head2 Subroutine, Signal, Sorting Traps
 
-=item *
+The general group of Perl4-to-Perl5 traps having to do with
+Signals, Sorting, and their related subroutines, as well as
+general subroutine traps.  Includes some OS-Specific traps.
 
-B<taintperl> is no longer a separate executable.  There is now a B<-T>
-switch to turn on tainting when it isn't turned on automatically.
+=over 5
 
-=item *
+=item * (Signals)
 
-Double-quoted strings may no longer end with an unescaped C<$> or C<@>.
+Barewords that used to look like strings to Perl will now look like subroutine
+calls if a subroutine by that name is defined before the compiler sees them.
 
-=item *
+    sub SeeYa { warn"Hasta la vista, baby!" }
+    $SIG{'TERM'} = SeeYa;
+    print "SIGTERM is now $SIG{'TERM'}\n";
+ 
+    # perl4 prints: SIGTERM is main'SeeYa
+    # perl5 prints: SIGTERM is now main::1
 
-The archaic C<while/if> BLOCK BLOCK syntax is no longer supported.
+Use B<-w> to catch this one
 
+=item * (Sort Subroutine)
 
-=item *
+reverse is no longer allowed as the name of a sort subroutine.
 
-Negative array subscripts now count from the end of the array.
+    sub reverse{ print "yup "; $a <=> $b }
+    print sort reverse a,b,c;  
+ 
+    # perl4 prints: yup yup yup yup abc
+    # perl5 prints: abc 
 
-=item *
+=back
 
-The comma operator in a scalar context is now guaranteed to give a
-scalar context to its arguments.
+=head2 OS Traps
+
+=over 5
+
+=item * (SysV)
+
+Under HPUX, and some other SysV OS's, one had to reset any signal handler, 
+within  the signal handler function, each time a signal was handled with 
+perl4.  With perl5, the reset is now done correctly.  Any code relying 
+on the handler _not_ being reset will have to be reworked.
+
+5.002 and beyond uses sigaction() under SysV
+
+    sub gotit {
+        print "Got @_... "; 
+    }  
+    $SIG{'INT'} = 'gotit';
+   
+    $| = 1;
+    $pid = fork;
+    if ($pid) {
+        kill('INT', $pid);
+        sleep(1);
+        kill('INT', $pid);
+    } else { 
+        while (1) {sleep(10);}
+    } 
+ 
+    # perl4 (HPUX) prints: Got INT...
+    # perl5 (HPUX) prints: Got INT... Got INT...
+
+=item * (SysV)
+
+Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does 
+the right thing w.r.t. the fopen() man page. e.g. - When a file is opened
+for append,  it  is  impossible to overwrite information already in
+the file.
+
+    open(TEST,">>seek.test");
+    $start = tell TEST ;  
+    foreach(1 .. 9){
+        print TEST "$_ ";
+    }
+    $end = tell TEST ;
+    seek(TEST,$start,0);
+    print TEST "18 characters here";
+ 
+    # perl4 (solaris) seek.test has: 18 characters here
+    # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
 
-=item *
 
-The C<**> operator now binds more tightly than unary minus.  
-It was documented to work this way before, but didn't.
 
-=item *
+=back
 
-Setting C<$#array> lower now discards array elements.
+=head2 Interpolation Traps
 
-=item *
+=over 5
 
-delete() is not guaranteed to return the old value for tie()d arrays,
-since this capability may be onerous for some modules to implement.
+=item * Interpolation
 
-=item *
+@ now always interpolates an array in double-quotish strings.
+
+    print "To: someone@somewhere.com\n"; 
+ 
+    # perl4 prints: To:someone@somewhere.com
+    # perl5 errors : Literal @somewhere now requires backslash
+
+=item * Interpolation
+
+Perl4-to-Perl5 traps having to do with how things get interpolated
+within certain expressions, statements, contexts, or whatever.
+
+Double-quoted strings may no longer end with an unescaped $ or @.
+
+    $foo = "foo$";
+    $bar = "bar@";
+    print "foo is $foo, bar is $bar\n";
+     
+    # perl4 prints: foo is foo$, bar is bar@
+    # perl5 errors: Final $ should be \$ or $name
+
+Note: perl5 DOES NOT error on the terminating @ in $bar
+
+=item * Interpolation
 
 The construct "this is $$x" used to interpolate the pid at that
-point, but now tries to dereference $x.  C<$$> by itself still
+point, but now apparently tries to dereference C<$x>.  C<$$> by itself still
 works fine, however.
 
-=item *
+    print "this is $$x\n";
 
-The meaning of foreach has changed slightly when it is iterating over a
-list which is not an array.  This used to assign the list to a
-temporary array, but no longer does so (for efficiency).  This means
-that you'll now be iterating over the actual values, not over copies of
-the values.  Modifications to the loop variable can change the original
-values.  To retain Perl 4 semantics you need to assign your list
-explicitly to a temporary array and then iterate over that.  For
-example, you might need to change
+    # perl4 prints: this is XXXx   (XXX is the current pid)
+    # perl5 prints: this is
+
+=item * Interpolation
+
+Creation of hashes on the fly with C<eval "EXPR"> now requires either both 
+C<$>'s to be protected in the specification of the hash name, or both curlies 
+to be protected.  If both curlies are protected, the result will be compatible
+with perl4 and perl5.  This is a very common practice, and should be changed
+to use the block form of C<eval{}>  if possible.
 
-    foreach $var (grep /x/, @list) { ... }
+    $hashname = "foobar";
+    $key = "baz";
+    $value = 1234;
+    eval "\$$hashname{'$key'} = q|$value|";
+    (defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");
+
+    # perl4 prints: Yup
+    # perl5 prints: Nope
+
+Changing
+
+    eval "\$$hashname{'$key'} = q|$value|";
 
 to
 
-    foreach $var (my @tmp = grep /x/, @list) { ... }
+    eval "\$\$hashname{'$key'} = q|$value|";
 
-Otherwise changing C<$var> will clobber the values of @list.  (This most often
-happens when you use C<$_> for the loop variable, and call subroutines in
-the loop that don't properly localize C<$_>.)
+causes the following result:
 
-=item *
+    # perl4 prints: Nope
+    # perl5 prints: Yup
 
-Some error messages will be different.
+or, changing to
 
-=item *
+    eval "\$$hashname\{'$key'\} = q|$value|";
+
+causes the following result:
+
+    # perl4 prints: Yup
+    # perl5 prints: Yup
+    # and is compatible for both versions
+
+
+=item * Interpolation
+
+perl4 programs which unconsciously rely on the bugs in earlier perl versions.
+
+    perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
+     
+    # perl4 prints: This is not perl5
+    # perl5 prints: This is perl5
+
+=item * Interpolation
+
+You also have to be careful about array references.  
+
+    print "$foo{"
+
+    perl 4 prints: {
+    perl 5 prints: syntax error
+
+=item * Interpolation
+
+Similarly, watch out for:
+
+    $foo = "array";
+    print "\$$foo{bar}\n";
+  
+    # perl4 prints: $array{bar}
+    # perl5 prints: $
+
+Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
+happy just to expand $foo to "array" by itself.  Watch out for this
+especially in C<eval>'s.
+
+=item * Interpolation
+
+C<qq()> string passed to C<eval>
+
+    eval qq(
+        foreach \$y (keys %\$x\) {
+            \$count++;
+        }
+    );
+  
+    # perl4 runs this ok
+    # perl5 prints: Can't find string terminator ")" 
 
-Some bugs may have been inadvertently removed.
+=back
+
+=head2 DBM Traps
+
+General DBM traps.
+
+=over 5
+
+=item * DBM
+
+Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
+may cause the same script, run under perl5, to fail.  The build of perl5
+must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
+to function properly without C<tie>'ing to an extension dbm implementation.
+
+    dbmopen (%dbm, "file", undef);
+    print "ok\n";
+
+    # perl4 prints: ok
+    # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
+
+
+=item * DBM
+
+Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
+may cause the same script, run under perl5, to fail.  The error generated
+when exceeding the limit on the key/value size will cause perl5 to exit
+immediately.
+
+    dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
+    $DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
+    print "YUP\n";
+
+    # perl4 prints:
+    dbm store returned -1, errno 28, key "trap" at - line 3.
+    YUP
+
+    # perl5 prints:
+    dbm store returned -1, errno 28, key "trap" at - line 3.
 
 =back
+
+=head2 Unclassified Traps
+
+Everything else.
+
+=item * Unclassified
+
+C<require>/C<do> trap using returned value
+
+If the file doit.pl has:
+
+    sub foo {
+        $rc = do "./do.pl";
+        return 8;
+    } 
+    print &foo, "\n";
+
+And the do.pl file has the following single line:
+
+    return 3;
+
+Running doit.pl gives the following:
+
+    # perl 4 prints: 3 (aborts the subroutine early)
+    # perl 5 prints: 8 
+
+Same behavior if you replace C<do> with C<require>.
+
+=back
+
+As always, if any of these are ever officially declared as bugs, 
+they'll be fixed and removed.
+