sub f{d,c,b,a,f,e}

##</code><code>##

my @list = (
     a => [ 'b', 'c', 'e' ],
     b => [ 'd' ],
     c => [ 'b', 'd' ],
     f => [ 'a' ],
);
print join (',', map { "'$_'" } f(@list)),"\n";