SVG is an XML application. Choose a module that can manipulate XML files and perform the necessary changes.

The module you pick may be dependent on what you're familiar with. I'd probably choose XML::LibXML because that's what I'm familiar with. You can search CPAN for modules in the XML:: namespace: there's a lot to choose from (and not all of them applicable to your current requirements).

From the limited information you've provided regarding the replacement, it appears that certain rect element attribute values need to be used (possibly after some manipulation) as image element attribute values. This looks straightforward, e.g. $href = substr($fill, 1) . '.svg', but come back and ask if you run into difficulties. [To get the best answers, follow the guidelines in "How do I post a question effectively?"]