Avoid $_, and all will be well. Try this:

use Tk;

use strict;
use warnings;

my $mw = MainWindow->new();
for my $id (0 .. 3) {
        $mw->Button(
            text => "Button$id", 
            command => sub { pub($id) }
        )->pack();
}
MainLoop;

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