A change from 5.8.x to 5.10.x is considered a change in major version for Perl 5. And unsurprisingly, in perl5100delta, the changes file listing the changes from 5.8.x to 5.10.0, there is mention:

Pseudo-hashes have been removed
  Support for pseudo-hashes has been removed from Perl 5.9. (The "fiel
+ds"
  pragma remains here, but uses an alternate implementation.)

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It seems that your use of fields is incompatible with the new implementation.

In reply to Re: Class::Struct::FIELDS broken under 5.10? by Corion
in thread Class::Struct::FIELDS broken under 5.10? by creeble

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