#!/usr/bin/env perl

use Modern::Perl;

sub rpow {
    my ($p, $q) = $_[0] =~ /(.)(.*)/;
    return $_[0] ? map { ($p.$_, $_) } rpow ($q) : '';
}

say join ("\n", rpow 'ABCD');

##</code><code>##

#!/usr/bin/env perl

use Modern::Perl;

sub bpow {
    map { my $m = unpack("b*", pack("i*",$_));
          $m =~ s/0/\0/g;
          $m =~ s/1/~"\0"/ge;
          $m & $_[0];
         } 0..2**length($_[0])-1
}

say join ("\n", bpow 'ABCD');