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";
                                                                                
}