Fix plperl build
The changes in
639928c988c1c2f52bbe7ca89e8c7c78a041b3e2 turned out to
require Perl 5.9.3, which is newer than our minimum required version.
So revert back to the old code for the normal case and only use the new
variant when both coverage and vpath are used. As the minimum Perl
version moves forward, we can drop the old code sometime.