use strict;
use warnings;

my $s = q<ÃÎÌËÁßÍÏÎÌÐÏÊÐÎÌßÏËÒÏÇÒÊÆßÓ�œž“ÏÑ–‘™�Áß‘ŠÆÌÇÌÑ‘ŠŒ�–�šÑœ�’ß�
+™>;
print pack "C*", map { $_ ^ 255 } unpack "C*", $s;
[download]

This only works if I run the code in an ASCII encoded file, there doesn't seem to be utf-8 involded here.

unpack "C*", $s; returns a list consisting of the value of each byte in $s. pack with "C*" takes a list of bytes and turns it into a string. Read perlpacktut for more information on that.