As perlvar says "A successful system or library call does not set the variable to zero.". Using $! without a system error is not meaningful.

For example, the perl interpreter could try to load a .pm file (like strict.pm) from different locations in @INC. It might not find it in the first directory it searches. A failure would set errno but a success would not reset it. I don't know for a fact if that is the exact cause of this particular error but my own guess. That would explain why not using strict does not set $! - no modules have been searched for.

Update: The difference in releases could just mean that the @INC is different and it finds the module in the first directory searched.