The one that pops to the top of my head is your C stack limit. The Perl regular expression engine is recursive, so if you write an RE that requires ridiculous depths of backtracking (on the order 10**3, depending on your system), you may blow your stack segment and segfault. Other than that, there's your (OS-imposed) open file limit, (until 5.8.0 comes out with PerlIO) your C runtime-imposed stdio limits (e.g., Solaris only allows 256 FILE* streams at once), and general memory limits. Oh, your line numbers are 16-bit integers, so they wrap at 2**16 ;-).