But I stand by my assessment of that usage as unusual, even strange

Outside of Perl, sure. In Perl, the second number has long been treated as the major version. This is especially true now that the top number can't be bumped without conflicting with the name of another language.

By (extreme) contrast, "Perl revision 6" finds 6 hits

I don't know why would you expect "Perl revision 6" to get matches. I'm surprised "Perl version 6" gets as much as 6% of "Perl 6" + "Perl6", though.

I also expect it to be quite common to read something like "Perl version 5.6.1"

Me too.