The minimal change solution:

defined(my      $MISC_FILE_DIR  = shift) || die usage();
#etc
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A possible alternative:

@ARGV == 5 or die usage();
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Your database problem is the same: there's a difference between undef and zero, but not to ||. If you have the // patch, you could use that. Otherwise, you need to call defined() instead of relying on the Boolean value of an expression.