... and I explained why - because you mix binary and text (upgraded) data.

If you don't want to switch template systems (foolish IMHO, but it's your choice) you can still make sure that all input to HTML::Template is binary data, not upgraded text strings. And make sure that no IO layer tries to encode them once again.

Or you can go the the bug tracker, and locally apply one of the two patches that add an open mode to HT.

Is there a particular reason you want to stick to a broken(*) template system? You don't have to change a single line in your templates if you use one that emulates HT's syntax.

(*): broken for this application. I once asked about Handling Encoding in Templates, and there was no viable solution offered for HTML::Template.