It looks like your (and my, and everyone else who has commented so far's) expectation of what "the image as a string" and "binary data" means doesn't jive with what it acctually means. Try this out and it works okay...

#!/usr/bin/perl

use strict;
use warnings;
use Tk;
use Tk::Photo;
use Tk::PNG;

my $file = shift or die "put a PNG file name on the command line";
my $mw = new MainWindow();
my $src = $mw->Photo(-format => 'png', -file => $file);
my $tgt = $mw->Photo(-format => 'png', -data => $src->data(-format => 
+'png'));
$mw->Label(-image => $tgt )->pack;

MainLoop;
[download]

Inspecting the output of src->data(-format => 'png') to understand what exactly the differnce is is left as an excercise for the reader.