Read "perldoc Tk::Error". You can either "require Tk::ErrorDialog" to redirect the error from stdout to a popup; OR you can define the sub Tk::Error to do what you want. Just return from sub Tk::Error to suppress any output.

#!/usr/bin/perl -w
use strict;
use Tk;
#require Tk::ErrorDialog;

my $mw = new MainWindow();

$mw->Button(-text=>'test',-command =>\&testit)->pack();

MainLoop;

sub testit{
  # division by zero error
  my $y = 1; my $x = 0; my $z = $y/$x;
 }

sub Tk::Error{ 
 my ($widget,$error,@locations) = @_;
 # print "$widget,$error,@locations\n";
 return; 
}
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