Probably, but this fact is unrelated with the question...

But it is directly related to your suggestions/answers/rambling

are we closer to the answer now that before your intervention?

No, the answer is as it was before, use http://search.cpan.org/perldoc?MediaWiki::API#MediaWiki::API-%3Elogin%28_$query_hashref_%29

Of course I'm open to new ideas about why this code in particular is not working

Maybe its because it is a fragment and it does nothing, maybe its because the credentials are wrong, maybe .... the error message says the credentials are wrong, so maybe the credentials are wrong