Indeed a big shining ray of sunshine ;). Sorry for the defensive behaviour I have asked for assitance with tasks on other similar sites and some people (I am sure you know the sort) were putting it plainly somewhat less then helpfull. Reading it again it does make me sound (justifiably) like a bit of a fool

Thankyou very much for all your assistance. You were quite correct my code was in fact not an infinite loop. However it did contain 2 very long arrays. The first array contained some 25000 elements and the second a rather large 46000 elements.

As a result the script that I wrote took 4 hours to run. Your own which you supplied me with I notice took about 5 seconds!!!! Your Perl kung-fu is mighty indeed Mr Antirice. Thankyou very much for your kind assistance and also the assitance of Aquarium and Skeeve.

There is one more twist in the tail. My coding problem was not resolved before I was sent the correct data file. It is a deeply embarressing thing to admit but I had been given the wrong data to attempt a comparison on. The files that I was given shared no common IDs. I failed on first principles before I had even begun. I trusted the data I was given. When I did recieve the correct file using your code I was able to resolve the problem very rapidly. For this again I thank you.