The following code loops endlessly:

use strict;
use warnings;
use File::Spec::Functions "devnull";

sub bar {
  open my $fh, "< $_[0]" or die "couldn't open $_[0]: $!";
  print while <$fh>;
}

sub foo { for (@_) { bar(devnull()) } }

$. = 0;
foo($.);
[download]

Do you see why? This is an especially bad variant on the common while-loop inside for smashing aliased $_ problem, which usually elicits a couple of points:

• Don't use for on $_ in a non-trivial script.
• Remember that a while-loop with implicit assignment to $_ doesn't localize $_; localize it yourself with local *_; (or my $_; in 5.10) (Note that local $_ does not fix the endless loop here).

But here, there's something else going on. A magic variable $. is being passed around; only its value is actually wanted, but instead, the container of the value is given. This can lead to trouble. Consider this case:

use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Terse=1;

sub foo {
   print "foo: bad args: ", Dumper \@_
      if @_ != 2 || $_[0] !~ /^[tf]/ || $_[1] !~ /statement/;

   my ($tf, $statement) = @_;

   if ($tf =~ /^t/) {  # Line 12
      print "true: $statement\n";
   } else {
      print "false: $statement\n"
   }
}

for my $statement ("the following statement is true",
                   "the preceding statement is false") {
   $statement =~ /\b(t|f|true|false)\b/ or next;
   foo($1, $statement);
}
[download]

Here, I am calling foo("true", "the following statement is true"); and foo("false", "the following statement is false");, but by the time foo gets to line 12, $tf is not set to the passed parameter. The container $1 was passed, and before it was assigned to $tf, an intervening regex undefined it. (However, because $1 is dynamically scoped, once foo() returns, $1 will again be "true" or "false".)

The moral here is to beware action at a distance; when passing magic variables, or even your own globals, if you want them passed by value, explicitly stringify or numify them:

   foo(0+$.);
   # and
   foo("$1", $statement);
[download]

so that the callee gets the value at the time of the call.

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