STDOUT Problem migrating a site to mod_perl

I have run into an odd problem when migrating a site to use mod_perl.

The script in question, builds some postscript into a variable, then 
uses psconvert (from Postscript::Convert) to change that into a png and 
sends that back to the webserver. (headers are set up earlier in the 
script).

This seems to work well with mod_cgi but doesn't work with mod_perl 
using either ModPerl::Registry or ModPerl::PerlRun.

Annoyingly psconvert doesn't return the converted image, instead it 
dumps it to a file (or stdout).

I have tried to re-implement this using GSAPI, but GSAPI doesn't seem to 
build on any of my machines. (all debian 9/10 based).

     use PostScript::Convert;
     psconvert(\$ps, '%stdout%' , format => 'png', resolution => '72', 
paper_size => '6.5x4');

I've searched through as much documentation as I can find, and tried 
various things, but none seem to work.

TIA for any guidance.