As long as the perl in question is built with uselargefiles=define, (which means all AS perls and most others by default for at least the last 6+ years, probably longer), then perl quite happily, accurately and correctly deals with files up to at least 9e15 (~9,000 Terabytes).

Also, I don't think it's even possible to get Win32::DirSize to give the size of a single file--unless it happened to be the only file in the directory--but in any case, it (and all it's conversion functions/methods) are totally unneccessary unless you have access to a filesystem that can store a single file > 9e15. (Is that even possible given current disk technology?)