base64.pl6

sub decode-base64(Str:D $str, Bool:D :$bin = False) {
    my @alpha     = flat 'A'..'Z','a'..'z','0'..'9', '+', '/';
    my %encodings = @alpha.kv.hash.antipairs;
    my $ords := $str.comb(/@alpha/).rotor(4, :partial).map: -> $chunk {
        my $n   = [+] $chunk.map: { (%encodings{$_} || 0) +< ((state $m = 24) -= 6) }
        my $res = (16, 8, 0).map: { $n +> $_ +& 255 }
        slip( $res.head( 3 - ( 4 - $chunk.elems ) ) );
    }
    ?$bin ?? Buf.new($ords || 0) !! $ords
}

my $str = decode-base64("dXNlcm5hbWU6dGhpc2lzbm90bXlwYXNzd29yZA==", :bin).decode;

exit $str eq "username:thisisnotmypassword" ?? 0 !! 1;

# say decode-base64("dXNlcm5hbWU6dGhpc2lzbm90bXlwYXNzd29yZA==")
# -> 2016.06: (117 115 101 114 110 97 109 101 58 116 104 105 115 105 115 110 111 116 109 121 112 97 115 115 119 111 114 100)
# -> 2016.07: (117 115 101 114 110 97 109 101 58 116 104 105 115 105 115 110 111 116 109 121 112 97 115 115 119 111 114 100)

# say decode-base64("dXNlcm5hbWU6dGhpc2lzbm90bXlwYXNzd29yZA==", :bin)
# -> 2016.06: Buf:0x<75 73 65 72 6e 61 6d 65 3a 74 68 69 73 69 73 6e 6f 74 6d 79 70 61 73 73 77 6f 72 64>
# -> 2016.07: Buf:0x<75 72 6e 61 6d 65 3a 74 68 69 73 69 73 6e 6f 74 6d 79 70 61 73 73 77 6f 72 64>
# Notes: 
#   - It always seems to lose the same two items (index 1 and 2)
#   - s/Buf.new($ords || 0)/Buf.new($ords)/ gives the correct results

Bisect log: https://gist.github.com/Whateverable/e0a28162b19f601616aa8776f24dd027

rakudo/rakudo@5581b24