Match comments better

This patch removes a bunch of warnings of the form:

    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 484, <$fh> line 165.
    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 485, <$fh> line 165.
    Use of uninitialized value $key in lc at /usr/share/perl5/site_perl/Rex/Config.pm line 487, <$fh> line 165.
    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 484, <$fh> line 119.
    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 485, <$fh> line 119.
    Use of uninitialized value $key in lc at /usr/share/perl5/site_perl/Rex/Config.pm line 487, <$fh> line 119.
    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 484, <$fh> line 165.
    Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/site_perl/Rex/Config.pm line 485, <$fh> line 165.
    Use of uninitialized value $key in lc at /usr/share/perl5/site_perl/Rex/Config.pm line 487, <$fh> line 165.