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.

Is that an example of raw input, or the output from the current iteration of your script? It would be helpful to know we have a clean fragment of raw logfile data