* Add test to demonstrate Perl syntax detection bug
A Perl 5 .pm file containing the word `module` or `class`, even with
an explicit `use 5.*` statement, is recognized as Perl 6 code.
* Improve Perl 5 and Perl 6 disambiguation
The heuristics for Perl 5 and 6 `.pm` files disambiguation was done
searching for keywords which can appear in both languages (`class` and
`module`) in addition to the `use` statement check.
Due to Perl 6 being tested first, code containing those words would
always be interpreted as Perl 6.
Test order was thus reversed, testing for Perl 5 first. Since Perl 6
code would never contain a `use 5.*` statement, this does no harm to
Perl 6 detection while fixing the problem to Perl 5.
Fixes: #3637
