If I run:

use Tk;

use strict;
use warnings;
my $mw = MainWindow->new();
for my $index (0 .. 10) {
    $mw->Button(-text=>"Button $index", -command => sub {print $index}
+)->pack();
}
MainLoop;
[download]

The variable $index is resolved at compilation time, so when I click a button it prints the number showing on the button. However if you replace $index with $_, like this:

use Tk;

use strict;
use warnings;
my $mw = MainWindow->new();
for (0 .. 10) {
    $mw->Button(-text=>"Button $_", -command => sub {print $_})->pack(
+);
}
MainLoop;
[download]

The first $_ resolves at compile time, and the the button displays the expected title. However the second $_ follows a different set of rule, and prints a warning (unintialized blah).

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