Here is one way.

    #!/usr/bin/perl -wl

    my $str1 = "1101000010101111";
    my $str2 = join "", unpack "b8b8", pack "B*", $str1;

    print for $str1, $str2;

    __END__
    Prints:

    1101000010101111
    0000101111110101
[download]

For this sort of task pack is usually the fastest method but to be sure you should benchmark it against the other solutions.

--
John.