Thanks for help. I discovered another possible source of the problems. The Perl binary on all the machines has the same md5sum, which means the 64-bit newer Linux runs the old 32-bit Perl binary compiled on a different machine and system. I'd guess it means all bets are off - there's no way how to fix it other than recompiling Perl from source. I was able to do that and the new 5.8.3 behaves correctly. Now, I just need to convince the managers we need to switch.