Or you could use glob.

my $pat = 'LLN';

my %pats = (
   L => [ 'a'..'z' ],
   N => [ '0'..'9' ],
);


($_) = map "{$_}", join ',', @$_
   for values %pats;

(my $glob = $pat) =~ s/(.)/$pats{$1}/g;


print("$_\n") for glob $glob;
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Update: I'm blind. I hadn't seen blokhead's glob solution (only his NestedLoops solution) before posting this.