I did notice its frequency is greater on runs of over 18000 iterations (Update: On further testing I found that theory not to hold up. The rest of the thread stands.</end update>), and even then it's about 30% of the time when I terminate by keyboard intervention.

Hmm, what other variables..... It's Windows Vista SP2. Perl version is 5.12.3, Strawberry Perl.

Here's another example run, where I terminated the loop earlier.

6043 is prime.
6047 is prime.
6053 is prime.
Out of memory!
panic: leave_scope inconsistency at C:\Users\Dave\Desktop\primes.pl li
+ne 15.
panic: leave_scope inconsistency at C:\Users\Dave\Desktop\primes.pl li
+ne 15.
panic: leave_scope inconsistency at C:\Users\Dave\Desktop\primes.pl li
+ne 15.
panic: leave_scope inconsistency at C:\Users\Dave\Desktop\primes.pl li
+ne 15.
[download]

And here's another example:

4409 is prime.
4421 is prime.
4423 is prime.
Out of memory!
Use of uninitialized value $i in concatenation (.) or string at C:\Use
+rs\Dave\Desktop\primes.pl line 18.
 is prime.
2 is prime.
Label not found for "next OUTER" at C:\Users\Dave\Desktop\primes.pl li
+ne 16.
[download]

...and one more...

6271 is prime.
6277 is prime.
Out of memory!
panic: leave_scope inconsistency at primes.pl line 15.
[download]