Checking for defined $_[1] may help.

From the docs describing the second parameter in the callback:

The characters to be added (or deleted). This will be undef if validation is due to focus, explcit call to validate or if change is due to -textvariable changing.

use strict;
use warnings;
use Tk;


my $page2;
my $w=0;
my $wtf;
my $wentry= 30;
my $wcb;

$page2 = MainWindow -> new();
$wcb = $page2 ->Checkbutton (-text=> '/W:', -variable=> \$w, -font=>"A
+dobe 10")->pack;
$wtf = $page2 ->Entry (-textvariable=> \$wentry, -width=>8,
            -validate=> 'key',
            -validatecommand=> \&CheckKey
)->pack;
MainLoop;

print "$w\n$wentry\n";

sub CheckKey
{
return 1 if ! defined $_[1];
return 1 if $_[1] =~/[0-9]/;
$page2->bell;
return 0;
}

[download]
Update: add explanation
Perl is Huffman encoded by design.

In reply to Re: Entry widget, default value and validatecommand by GrandFather
in thread Entry widget, default value and validatecommand by Real Perl

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