still, it could be reasonable to write simple test script to open that dir and just count: 
opendir DIR, "that-dir";
my @files = readdir DIR; # or whatever methos Mail::Box uses
print "count is ", $#files+1;

[download]
Just to check if perl's readdir is out of question on your current build (with given libc and so on)

In reply to Re^3: Mail::Box fails miserably when trying to open 30_000 messages maildir by Courage
in thread Mail::Box fails miserably when trying to open 30_000 messages maildir by monsieur_champs

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