Here's a little info that will help you understand Perl/Tk programs:

Perl/Tk applications are event driven, which means that the program waits for "events" (keyboard, mouse, etc) and then executes accordingly. Event driven programs have a loop somewhere in them that catches and processes these "events". This is what MainLoop in Tk is. It is simply a while loop that catches and processes events. Consequently, the code after MainLoop never gets executed, unless it is

1) part of a sub that is called before MainLoop executes or
2) part of a callback attached to a Tk widget.

This is why print @a; never prints. To get it to print, put it in a sub that is part of a callback. Something like:

$mw->Button(-text => "print", -command=> sub {print @a })->pack;
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or

$mw->Button(-text => "print", -command => \&print_it);
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then somewhere below MainLoop

sub print_it() {
    print @a;
}
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For a better explanation of Perl/Tk programming I would suggest that you get the book Mastering Perl/Tk.

Hope this helps,
davidj