package My::Tk::DialogBox;
use base qw/Tk::DialogBox/;
sub Show {
  # your version of Show() here.
}
package main;
my $dialogbox = My::Tk::DialogBox->new( ... );

##</code><code>##

package Tk::DialogBox;
sub Show {
  # your version of Show() here.
}
package main;
my $dialogbox = Tk::DialogBox->new( ... );