It gets tricky on the validate command, because it dosn't do exactly what you think. This example shows what it actually does, and gives a possible solution. Some clever regex guru may be able to make a regex, but this works.

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

my $mw = MainWindow->new();

my $content = 'go ';

my $entry = $mw->Entry( -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-> $changedchars\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;
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