#!/usr/bin/perl -w
                                                                                
use Tk;
use strict;
                                                                                
my $mw = MainWindow->new;
                                                                                
    my $var=0;
                                                                                
    my $button_A = $mw->Button(
    -text    => '  Button A',
    -command =>[\&rutine, \$var, ]);#
    $button_A->pack;
                                                                                
    my $button_B = $mw->Button(
    -text    => ' ++ ',
    -command => sub{$var++;  print "\$var is $var!\n";});
    $button_B->pack;
                                                                                
    my $button_C = $mw->Button(
    -text    => ' -- ',
    -command => sub{$var--; print"\$var is $var!\n";});
    $button_C->pack;
                                                                                
MainLoop;
                                                                                
                                                                                
sub rutine
{
    my ($pvar)=@_;
                                                                                
    print "\$pvar => $$pvar\n";
                                                                                
}

##</code><code>##

    my $button_A = $mw->Button(
    -text    => '  Button A',
    # -command =>[\&rutine, \$var, ]);#
    -command => sub{ rutine($var) });
    $button_A->pack;

    # ...

sub rutine
{
    my ($var)=@_;
                                                                                
    print "\$var => $var\n";
                                                                                
}