We don't use as much perl as we used to but the last upgrade issue was amavisd-new (a Spamassassin wrapper). Spamassassin has perl version issues every so often as well. NetDNS used to introduce new bugs about every 4th revision but seems to have been stable for the past couple of years. GNUmp3d and many audio libraries have non-perl revision-related, backwards-compatibility issues with some regularity.