Another alternative which you may be overlooking, is to forget the -validate stuff, and just bind on the Return keypress, so you get the whole string at once. Then you can regex it.

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

my $mw = MainWindow->new();

my $content = '';
my $entry = $mw->Entry(-textvariable => \$content)->pack();

$entry->bind('<Return>',sub{ 
   my $val = $entry->get;
   print "check with regex here and do what is needed  $val\n" });

$entry->focus;
MainLoop;
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