use Math::Combinatorics qw( permute );

my @charset = qw( a b c );
my %valid = map { join( '', @$_ ) => 1 } permute( @charset );
my ($others) = map qr/[^$_]/, join '', map quotemeta, @charset;

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