$ perl5.8.6 -MRegexp::Common=list -e 'print map {$_.$/} grep { /$RE{list}{-pat=>"[ugoa]+=[rwxsS]+"}{-sep=>","}/ } @ARGV' ugo=rw,a=r ugo=rw
ugo=rw,a=r

##</code><code>##

my $perm_re = qr/[ugoa]+=[rwxsS]+/;
$val =~ /^(?:$perm_re(?:,$perm_re)*)$/;