GatherDir uses exclude_filename as a regex, causing files to be excluded incorrectly [rt.cpan.org #81492]

[rjbs]

https://rt.cpan.org/Ticket/Display.html?id=81492

For example, try excluding MANIFEST while including MANIFEST.SKIP. 
Because the match is done as an unanchored regex, the latter will be 
incorrectly excluded. If exclude_filename is really going to be a regex, 
then what's the point of match? To match the behaviour described in the 
docs, the match must be against the whole filename.