Hi, I think what you are looking for is the Entry's validate option. Check out how the following example works.

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

my @vars;

my $mw  = tkinit;
my $row = 0;
$mw->Label( -text => 'Unit cost:', )
 ->grid( -row => $row, -column => 0, -sticky => 'e' );
$mw->Entry(
   -textvariable    => \$vars[ 0 ],
   -validate        => 'key',
   -validatecommand => sub { multiply( $vars[ 1 ], @_ ) },
)->grid( -row => $row++, -column => 1, -sticky => 'w' );

$mw->Label( -text => 'Quantity:', )
 ->grid( -row => $row, -column => 0, -sticky => 'e' );
$mw->Entry(
   -textvariable    => \$vars[ 1 ],
   -validate        => 'key',
   -validatecommand => sub { multiply( $vars[ 0 ], @_ ) },
)->grid( -row => $row++, -column => 1, -sticky => 'w' );

$mw->Label( -text => 'Total cost:', )
 ->grid( -row => $row, -column => 0, -sticky => 'e' );
$mw->Entry( -textvariable => \$vars[ 2 ], )
 ->grid( -row => $row++, -column => 1, -sticky => 'w' );

sub multiply {
   my ( $var, $proposed ) = @_;
   my $product;
   eval { $product = ( $var || 0 ) * ( $proposed || 0 ) };
   $product = 0 if $@;
   $vars[ 2 ] = $product;
   return ( 1 );
}
MainLoop;
[download]