The killer is the file associations (assoc) which tie the .pl extension to a specific executable.

Yes, but what about associating .pl with the new version (i.e. make it the default), and re-associating things back in case you should encounter problems.

I'm only an occasional Windows user, but this seems rather straightforward to me — at least I can't remember to have ever had any problems with multiple Perl versions on Windows.  And precompiled Windows Perl installations have "always" been relocatable to an arbitrary top-level directory (at least the ActiveState versions), while that feature has not been available on Unix before Perl 5.10.