See the Tk::Entry documentation. In particular, the -validate, -validatecommand and -invalidcommand options (in WIDGET-SPECIFIC OPTIONS) and the entire VALIDATION section.

Also see the "Validated entries and password fields." examples in the Widget Demo.

I think I made some progress:

     $search = $top->Entry ('-width'            => 20,
                            '-validate'         => 'key',
                            '-validatecommand'  => sub {$_[1] =~/\w+/ 
+;} ,
                            '-invalidcommand'   => sub {$top -> bell()
+ ;},
               ) ->pack('-side' => 'left');
[download]

I found the code on 'Mastering Perl/Tk' -- I modified it to work with my program. But I still need it to accept ':' and '-'. As for invalid input it should do something like this:

'-invalidcommand' => sub {$_[1] =~/^[[:cntrl:]] ;
                          $top -> bell;
                         }
[download]

But it sill locks up...

2021-03-09 Athanasius added code tags around final snippet.

You could use a character class to accept : and - in the validatecommand sub.

'-validatecommand'  => sub {$_[1] =~/[\w:-]+/ ;} ,
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WRT your invalidcommand sub, the regexp does nothing at the moment. Should the first line be something like this if you want to accept control characters?

'-invalidcommand' => sub {
                          return if $_[1] =~/^[[:cntrl:]] ;
                          $top -> bell;
                         }
[download]

Or do you want the bell if it starts with a control character?

'-invalidcommand' => sub {
                          $top -> bell if $_[1] =~/^[[:cntrl:]] ;
                         }
[download]

I don't code with Tk, but hopefully this is useful.

If $search->get() is returning undef on <RET> then change the condition in the goto line so it avoids an infinite loop.

It's probably also useful to autoflush stdout to see the feedback as it is generated, rather than once the buffer is sufficiently full. See https://perldoc.perl.org/perlvar#$%7C.

#  somewhere in your code, perhaps in the search sub itself.
local $| = 1;
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#!/usr/bin/perl

use strict;
use warnings;
use Tk;
$| = 1;

my $search_pattern = '';

my $mw = MainWindow->new;
$mw->geometry( '+700+300' );
my $search = $mw->Entry (-width => 20, -textvariable => \$search_patte
+rn)->pack;
$search->bind('<KeyPress-Return>', \&search);
$search->focus;
$mw->Button(-text => 'Clear', -command => sub {$search_pattern = ''})-
+>pack;
$mw->Button(-text => 'Exit', -command => sub {$mw->destroy})->pack;

MainLoop;

sub search
  {
  my ($string );
  print "Pattern entered into Search-box is:  $search_pattern\n   ";
  use Data::Dump 'dd'; dd 'got', [ $search_pattern ];
  ## CHECK $search_pattern and untaint:
  if( $search_pattern =~ /^([\w\s\-\:\_\d]+)$/ )
    {  ## make sure it only contains \w, '-' , ':' , '_' , and digits
    $string = $1; ## DON'T forget 's -- space between prog and $arg
    print "\$string is untainted; \$string = $string \n ";
    }
  else
    {
    print "OOPS! data is tainted or empty. TRY AGAIN...\n  ";
    }
  }
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I just noticed the code you posted. I tried it - it works the way I want. I am gonna play with it, and see if it can be added to my program. Thanks.

I got your code incorporated. Your code is quite simple. I am working on something thats quite large (1000 lines). Should $| be at the beginning of the program? If I enter '[' in the Tk::Entry widget I get "stack moved" error. If I promptly hit <RET> it still freezes. I pass data to $search_pattern via <code? $search_pattern = $entry -> get(); </code> Ideas?

Your immediate problem is simple: sub search goes into an infinite loop if $search_pattern is undefined. Instead of a goto, simply return early, as in return unless defined $search_pattern; which will cause the Tk event loop to resume and the UI to wait for the user to try again.