#! /usr/bin/perl
use warnings;
use strict;
use Tk;
my $entryText = 1;
print "Perl-Version: $]\nTk-Version: $Tk::VERSION\n";

my $mw = MainWindow->new();

my $entry = $mw->Entry( -textvariable    => \$entryText,
                        -validate        => 'all',
                        -validatecommand => \&ValidateCmd,
                       )
    ->pack( -side => 'left' );

$mw->Button( -text    => 'increase',
             -command => sub { $entryText++ },
            )
    ->pack( -side => 'right' );

MainLoop;

sub ValidateCmd {
    my( $newValue ) = @_;
    {   local $" = '] [';
        no warnings 'uninitialized'; print "ARGS: [@_]\n";
    }

    # return error if value > 3 and don't change the content
    if( $newValue > 3 ) {
        print "Error: Too big: $newValue\n";
        return 0;
    } # if

    return 1;
} # ValidateCmd