The mailbox was checked once every 30 minutes with a script that ran for 30 seconds, so simultaneous file access was high unlikely--but not impossible.

Hmmm. If you are 'reading' a mailbox, as a mail client might, then it seems you wouldn't run into the need to lock the file.

I'd would approach the problem like so:

my $mboxfhandle;

# open $mboxfhandle for readonly

my @mbox = <$mboxfhandle>;  # Let's use lots of memory

# rest of the process ...
[download]

Seems like this would catch each new message, and not conflict with any incoming messages.

( on further reflections, I imagine that a mail client must be able to mark messages as read, which would involve a write ... )

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