This will do it, without creating any temp image file. You need to get the binary image with LWP, Base64encode it to please Tk, then use the -data option.

#!/usr/bin/perl -w
use strict;
use Tk;
use Tk::JPEG;
use LWP::Simple;
use MIME::Base64;

my $URL = 'http://zentara.net/2uni2.jpg';
my $content = encode_base64(get($URL)) or die $!;

my $mw  = MainWindow->new();
my $image = $mw->Photo(-data => $content);
$mw->Label(-image => $image)->pack(-expand => 1, -fill => 'both');
$mw->Button(-text => 'Quit', -command => [destroy => $mw])->pack;
MainLoop;
############################################# 
#1 liner 
# perl -MLWP::Simple -MMIME::Base64 -MTk -MTk::JPEG -e '$mw=tkinit; 
#  $mw->Label(-image => $mw->Photo(-data => encode_base64(get(shift)))
+)->pack;MainLoop'  
#   http://...
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