Can you show us the code you wrote using PDF::Reuse? It looks like the prAdd method will allow images to be added to existing pages.

You may also need to use something like ghostscript for that level of PDF manipulation. There is a GSAPI module at CPAN that provides Perl access to the ghostscript API, but I've never used it personally.

There's a PDF FAQ here. Your question of how to add watermarks is covered there but they suggest using this tool, which you have to buy.