"... the syntax ... is quite unfamiliar to me."

The fix for that would be to learn the language and familiarise yourself with the syntax.

"I rather suspect that Perl didn’t understand use set my $y =666; at all, ..."

You could have just run some very simple tests to determine the validity of that.

$ perl -Mwarnings -Mstrict -E 'say "@INC"'
/some/path /other/path ...

$ perl -Mwarnings -Mstrict -E 'use lib; say "@INC"'
/some/path /other/path ...

$ perl -Mwarnings -Mstrict -E 'use lib my $x = "fred"; say "@INC"'
fred /some/path /other/path ...
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I've downvoted your post.