my @set = qw(a b c);

my %uniq;
my @comb = grep !$uniq{$_}++, map join(",",sort split(//,$_)), glob(("{".join(",",@set)."}") x @set);

print "$_\n" for @comb;

__END__
a,a,a
a,a,b
a,a,c
a,b,b
a,b,c
a,c,c
b,b,b
b,b,c
b,c,c
c,c,c