Absolutely right japhy, but some might find your explanation a bit hard to swallow. It's a complicated topic, so I'll try to illustrate it.

---

The minimal code illustrating the problem is:

# Module.pm
use warnings;
package Module;
my $thingy = 'thangy';
sub doit { eval 'print "thingy is $thingy\n"'; }
1;
[download]

use Module;
Module::doit();  # "thingy is "
[download]

Each Perl files (both scripts and modules) create a scope. Just like adding {...} around statements creates a scope for the contained statements, use, require, etc create a scope in which the file is executed.

The means the above code is equivalent to:

{
   use warnings;
   package Module;
   my $thingy = 'thangy';
   sub doit { eval 'print "thingy is $thingy\n"'; }
   1;
}

Module::doit();  # "thingy is "
[download]

Let's simplify the above to:

{
   use warnings;
   my $thingy = 'thangy';
   sub doit { eval 'print "thingy is $thingy\n"'; }
}

doit();  # "thingy is "
[download]

When Perl reached the "}", it doesn't realize you'll still need $thingy (since the eval hasn't run yet), so $thingy is freed. If you added use strict; to the string being evaled, you'd get a strict error. eval's $thingy refers not to the lexical, but to the package variable $main::thingy.

Enter closures. Closures force variables to stick around longer than they normally would:

{
   use warnings;
   my $thingy = 'thangy';
   sub doit { print "thingy is $thingy\n"; }
}

doit();  # "thingy is thangy"
[download]

So you need to add a reference to $thingy in your sub so Perl knows (at compile-time) that $thingy is still needed:

{
   use warnings;
   my $thingy = 'thangy';
   sub doit { my $ref = \$thingy;  # Close over $thingy
              eval 'print "thingy is $$ref\n"'; }
}

doit();  # "thingy is thangy"
[download]

By "reference", I meant "occurance". In the previous snippet, my reference to $thingy is in the expression creating a reference to $thingy, but it need not be so:

{
   use warnings;
   my $thingy = 'thangy';
   sub doit { my $thingy = $thingy;  # Close over $thingy
              eval 'print "thingy is $thingy\n"'; }
}

doit();  # "thingy is thangy"
[download]

Alternatively, use package variables instead of lexical variables. They don't care about scope:

{
   use warnings;
   our $thingy = 'thangy';
   sub doit { eval 'print "thingy is $thingy\n"'; }
}

doit();  # "thingy is thangy"
[download]

---

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