Ok, latest summary...

It appears that the error does not occur unless both Data::Dumper and /x are present. Since the error is so sporadic, it is hard to be sure.

NB Just avoiding the use of Data::Dumper (say by using Dumpvalue instead) is not an option, since many CPAN modules which I am using themselves use Data::Dumper.

I will hang on a bit longer, to see if anybody spots anything else, then submit a bug report.

Cheers, Kevin