use Math::Combinatorics qw( permute );
use Regexp::List qw( );

my @charset = qw( a b c );
my ($valid) = map qr/^$_\z/, Regexp::List->new()->list2re(permute(@charset));
my ($others) = map qr/[^$_]/, join '', map quotemeta, @charset;

sub match {
   my $s = shift;
   $s =~ s/$others//g;
   return $s =~ $valid;
}