"30000" smells like "signed 16-bit overflow". This problem may even be in an underlying system library (because Mail::Box doesn't use XS).

I'd start by investigating which mail box format the production environment uses.

In reply to Re: Mail::Box fails miserably when trying to open 30_000 messages maildir by fglock
in thread Mail::Box fails miserably when trying to open 30_000 messages maildir by monsieur_champs

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