As JediWizard identified, using $_ as your id is the source of the problem, though his solutions aren't quite right.

In his first example, quoting $_ won't do the trick, and in the second the quoting is unnecessary.

You could also use your current code (with corrections:), but name the loop iterator. By naming the iterator, the sub will form a closure over the lexical and achieve the results you want.

#! perl -slw
use strict;
use Tk;
use Data::Dumper;
use XML::Simple;

use strict;
use warnings;

my $tasks = XMLin(<<EOS);
<tasks>
    <task description="task1"></task>
    <task description="task2"></task>
    <task description="task3"></task>
    <task description="task4"></task>
    <task description="task5"></task>
    <task description="task6"></task>
</tasks>
EOS

print Dumper($tasks);

my $mw = MainWindow->new();

## What was $#{@{$tasks->{"task"}}} all about :)
for my $id ( 0 .. $#{ $tasks->{task} } ) { 
    $mw->Button( 
        -text => $tasks->{task}[$id]{description}, 
        -command => sub { pub( $id ) }
    )->pack();
}
MainLoop;

sub pub {
        my $task_id = shift;
        print "task id = $task_id";
}

__END__
P:\test>junk
$VAR1 = {
          'task' => [
                    {
                      'description' => 'task1'
                    },
                    {
                      'description' => 'task2'
                    },
                    {
                      'description' => 'task3'
                    },
                    {
                      'description' => 'task4'
                    },
                    {
                      'description' => 'task5'
                    },
                    {
                      'description' => 'task6'
                    }
                  ]
        };

task id = 0
task id = 1
task id = 2
task id = 3
task id = 4
task id = 5
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