First, it's probably a good idea to upgrade your Perl, if anyhow possible. In particular, if you need unicode support. For one, 5.6.1 had quite a different approach to unicode internally than current 5.8 versions. Secondly, it's rather ancient anyway, so you're less likely to receive community help with problems that are specific to 5.6.   Just as an example, the binmode STDOUT, ':utf8'; mentioned by Errto does not work in 5.6 (you'd get an "Unknown discipline ':utf8' ..." error).

Apart from that, it would be interesting to know in what encoding you have specified your greek 'y' in $string1. Depending on that, you probably want to add a use encoding ... directive (also 5.8, btw) to tell Perl how to parse literal strings, regexes, etc. in the script itself (e.g. use encoding 'utf8'; if the script is encoded in UTF-8).