It's not clear to me how you did the "sub-classing", by inheriting methods or by importing or a do or ...?

Anyway, as I already said, I prefer monkey-patching over subclassing and you won't need to create another module/namespace for it.

Sub-classing requires that the author played by "the" book, and the OOP book is indeed a TIMTOWTDI library in Perl.

Monkey-patching OTOH is well defined and has no dependency to any OOP model.

So in our own interest consider

sub call_patched {

    local *Spreadsheet::ParseXLSX::new             = \&patched_new;
    local *Spreadsheet::ParseXLSX::_parse_workbook = \&patched_parse_w
+orkbook;
   
    # do whatever with Spreadsheet::ParseXLSX
}
[download]

And all patches will disappear outside this function.

For completeness: Both ways may fail if there are any lexical variables closed over the patched subs, but even this can be solved with PadWalker, iff this really happens.