In the actual code, I still create a GD::Image so that I can read the width

No need. PDF::API2 has methods to get image size. + Using PDF::API2::image_whatever directly is safe, BTW.

use strict;
use warnings;
use feature 'say';
use open IO => ':raw';
use GD;
use PDF::API2;

my $gd_obj = GD::Image-> new( scalar qx/convert rose: png:-/ );
my $pdf = PDF::API2-> new;
my $pdf_image = $pdf-> image_gd( $gd_obj );

say $pdf_image-> width;
say $pdf_image-> height;
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