my (%lookup, @mapping);
CHAR:    
for my $char (split //, $sh_str) {
    my @chr_pos_all_strs;
    for (0 .. $#pos) {
        # Ignore chars not in every string
        next CHAR if ! $pos[$_]{$char};

        # All slots to be variable number of positions
        push @chr_pos_all_strs, $pos[$_]{$char};
    }
    my $next = NestedLoops([@chr_pos_all_strs]);

    # Generate all combinations 
    while (my @char_map = $next->()) {
        # stringified reference to convert back to char later 
        my $ref = [@char_map];
        $lookup{$ref} = $char;

        push @mapping, $ref;
    }
}