Drop the leading "perl-" from perlbrew'ed perls, if present

Also, we can directly use our Perl icon as we're always going to have a Perl
prompt segment (though this may change in the future).
change our "git describe" icon; strip "heads/" if present

...as we pretty much know it's going to be out of heads/ anyways; that's only
really valuable when we get describe answering with tags/etc, anyways.