sub interleave {
    # PNAMBIC; I'm just making two arrays using length args
    @a = split //, "A" x shift;
    @b = split //, "B" x shift;
    
    # Some magic happens here with @$a and @$b
}

##</code><code>##

# These are just demonstrator edge cases
interleave(1, 1);
AB
interleave(2, 1);
ABA
###  This is the good stuff
interleave(1, 2);
ABB        # BAB and BBA are also fine
interleave(2, 2);
ABBA
interleave(3, 3);
ABABBA     # ABBABA is also fine
interleave(4, 14);
ABBBBABBBBBABBBBBA  # 'B' strings can be swapped around
interleave(7, 4);
ABABABABAAA