There are some numbers in computing, which are boundries, and can cause problems (sort of like that whole y2k issue).
Near 30,000 is the number 32,768, which is 2**15. Now, you'd think to yourself, but wouldn't there be problems at 2**16, which is a nice round number in computer terms?
Well, no, because for an integer of (x) bits, if it's signed, it ranges from (-1*(2**(x-1)-1) to 2**(x-1). So, for a 16 bit number, it goes from -32767 to 32768. If the module in question uses XS (compiled C code), it's possible that it was compiled with a 16 bit signed number in there, which will have problems if you try to deal with numbers greater than 32,768.
If the number is exactly 30,000 or less, this probably isn't the issue. If it's over 32,768, this could be a problem.
From looking at the docs for Mail::Box, however, it looks to be pure perl, so I don't think this is the issue in this case.