sub watch_variable
{
	my( $mainwindow, $scalarref, $coderef ) = @_;
	$mainwindow->Entry(
		-textvariable => $scalarref,
		-validate => 'all',
		-validatecommand => sub { $coderef->( $_[0] ); 1 },
	);
}


##</code><code>##


use Tk;
use strict;
use warnings;

my $mw = new MainWindow;

my $var;

watch_variable( $mw, \$var, sub { warn "val = $_[0]\n" } );

$mw->Button( -text => "Incr", -command => sub { $var++ } )->pack;
$mw->Button( -text => "Set 5", -command => sub { $var = 5 } )->pack;
$mw->Button( -text => "Exit", -command => sub { exit } )->pack;

MainLoop;

sub watch_variable
{
	my( $mainwindow, $scalarref, $coderef ) = @_;
	$mainwindow->Entry(
		-textvariable => $scalarref,
		-validate => 'all',
		-validatecommand => sub { $coderef->( $_[0] ); 1 },
	);
}