use strict;
use warnings;
use Tk;

my $mw = MainWindow->new;

my $firstentry = $mw->Entry(
    -validate => 'focusout',
    -vcmd     => [ \&check_state ],
)->pack();
my $secondentry = $mw->Entry(
    -validate => 'focusout',
    -vcmd     => [ \&check_state ],
)->pack();

my $but = $mw->Button( -text => "OK", -state => 'disabled' )->pack();

MainLoop;

sub check_state {
    if ( $firstentry->get and $secondentry->get ) {
        $but->configure( -state => 'normal' );
    }
    else {
        $but->configure( -state => 'disabled' );
    }
}