This is a regression in perl 5.10.0, caused by http://perl5.git.perl.org/perl.git/commitdiff/800401ee2a8a5a67ef478227b68426cf701d0116 . The attached patch fixes this. I’m not sure whether what I’ve done to sv_2nv is correct. Could someone please review it? If anyone needs a workaround for this bug in the mean time, I’ve added fix_tie to Tie::Util.Thread Next