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

my $mw = MainWindow->new();

my $content = 'go ';

my $entry = $mw->Entry( 
            -bg => 'white',
            -font => 24,
            -textvariable => \$content , 
            -validate => 'key' ,
            -validatecommand => sub { 
                                      
                    my $newvalue     = shift;
                    my $changedchars = shift;
                    my $currentvalue = shift;
                    my $index        = shift;
                    my $type         = shift;
                    
                    print "newval-> $newvalue\nchangedchars-> $changed
+chars\n".
                          "curvalue-> $currentvalue\n index-> $index\n
+";
                    
                    if( ($index == 0) and ($newvalue ne 'g')){return 0
+}                    
                    if( ($index == 1) and ($newvalue ne 'go')){return 
+0}                    
                    if( ($index == 2) and ($newvalue ne 'go ')){return
+ 0}                       
                       
                       return 1;
            
                        },
            
            -invalidcommand => sub { print "ERROR.\n", $mw->bell() } 
            
            
            )->pack() ;

$entry->icursor(3);
$entry->focus;

MainLoop;
[download]

Try Tk::MatchEntry.

Regards, Ştefan

I know this will involve some sort of bind + callback, but I cannot figure out how to do it...

You can always look it up :) site:perlmonks.org Tk "bind(" text entry -> Tk::Entry selection problem

You can also find examples with

$ perl -MTk -le " print Tk->findINC() "
C:/perl/site/5.14.1/lib/MSWin32-x86-multi-thread/Tk

$ ack --match "->bind" C:/perl/site/5.14.1/lib/MSWin32-x86-multi-threa
+d/Tk
...
[download]