From the documentation, it doesn't look as if ->image_jpeg accepts an URL. You could check that by checking the return value:

my $imagename = "http://docs.gimp.org/images/filters/examples/color-ta
+j-sample-colorize.jpg";
my $jpeg = $pdf->image_jpeg($imagename)
    or die "Couldn't add '$imagename' as a file.";
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