To make up for my ignorance:

#!/usr/bin/perl
use strict;

use Tk;
use Tk::Photo;

my $main = new MainWindow;

my $photo = $main->Photo(-format => 'gif', -file => 't.gif');

my $canvas = $main->Canvas(
    -width => $photo->width,
    -height => $photo->height,
)->pack( -side => 'top' );

$canvas->createImage(0, 0, -image => $photo, -anchor => 'nw');

MainLoop;
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Tk::Photo is not a widget, just an object that contains the photo. Use a Tk::Canvas to display the photo.