use Image::Grab qw/grab/;

my $imgdata = grab('http://upload.wikimedia.org/wikipedia/commons/1/1a/Image_upload_test.jpg');
# - or -
my $pic = Image::Grab->new;
$pic->url('http://upload.wikimedia.org/wikipedia/commons/1/1a/Image_upload_test.jpg')
$pic->grab;
my $imgdata = $pic->image;