Fix perl-reversion to not match with v-prefix in "pre"

If you have a version line like:

    $VERSION = version->declare('v0.1.2')

perl-reversion seems to swallow the 'v' part in its version_re_perl. This commit changes the match with '?' in its regexp so that Perl::Version::REGEX can correctly capture the 'v' part.

Realistically, perl-reversion works fine without this patch, since handling the 'v' part as a pre-text causes no troubles, but this patch feels more correct.