The easiest way to do it would probably be by using LWP::Simple (see 'get') and Image::Size (using 'imgsize'):

#!/usr/bin/perl -w

use Image::Size;
use LWP::Simple;

use Carp;

use strict;

{
  my $loc = "http://perlmonks.org/images/usermonkpics/httptech.gif";

  my $img = get $loc or croak "Can't get $loc!";

  # when the argument passed to imgsize
  # is a reference (as opposed to a
  # simple scalar), it assumes it's
  # pointing to an in-memory buffer,
  # instead of to a filename.

  my ($width, $height) = imgsize(\$img); 

  print "Width: $width\n", "Height: $height\n";
}
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