Go source diving :) Bug #78121 for HTML-Template-Pro: I found a memory leak

If you can find the origin of the leak in c/xs land, you might be able to fix it, then it might work with more than 8k for you (I assume the two bugs are related, otherwise its all c programming stuff)

OTOH, you could try HTML::Template::Parser - parser for HTML::Template::Pro and writer to Text::Xslate.
https://metacpan.org/source/SHMORIMO/HTML-Template-Parser-0.1011/script/html-template-converter.pl

Good luck