document incompatible perl4 vec() vs bitwise ops interaction trap

(from Tom Phoenix)

p4raw-id: //depot/perl@4630

1 files changed, 24 insertions, 0 deletions

diff --git a/pod/perltrap.pod b/pod/perltrap.pod
index 50987cb102..4920f538c3 100644
--- a/pod/perltrap.pod
+++ b/pod/perltrap.pod
@@ -715,6 +715,30 @@ Logical tests now return an null, instead of 0
 Also see L<"General Regular Expression Traps using s///, etc.">
 for another example of this new feature...
 
+=item * Bitwise string ops
+
+When bitwise operators which can operate upon either numbers or
+strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
+treat the operands as bitstrings so long as the program contained a call
+to the C<vec()> function. perl5 treats the string operands as bitstrings.
+(See L<perlop/Bitwise String Operators> for more details.)
+
+    $fred = "10";
+    $barney = "12";
+    $betty = $fred & $barney;
+    print "$betty\n";
+    # Uncomment the next line to change perl4's behavior
+    # ($dummy) = vec("dummy", 0, 0);
+
+    # Perl4 prints:
+    8
+
+    # Perl5 prints:
+    10
+
+    # If vec() is used anywhere in the program, both print:
+    10
+
 =back
 
 =head2 General data type traps