$pl= ExtUtils::Packlist->new([plfile])
Constructor. Takes the name of a .packlist as the optional parameter
and returns a reference to a hash that has an entry for each line in the
.packlist. The value associated with each key is undef for
oldstyle .packlists. For files in the extended format, the value
for each key is a hash containing the key/value pairs on the line associated
with that key.