Also you can check out perldoc Tk::Derived. Here is an example:

#!/usr/bin/perl
use Tk;
use strict;

package MyButton;

use base qw/Tk::Derived Tk::Button/;
Construct Tk::Widget 'MyButton';

sub ClassInit {
     my ($class, $mw) = @_;
     $class->SUPER::ClassInit($mw);
}

sub Populate {
     my ($self, $args) = @_;
     $self->SUPER::Populate($args);
     $self->bind('<Enter>', sub{print "Entered a MyButton\n"});
}

1;

package main;

my $mw = MainWindow->new;
$mw->Button(-text => 'NormalButton')->pack;
$mw->MyButton(-text => 'MyButton')->pack;

MainLoop;
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Also check the Tk::mega manpage for more information.