Thank you all so much for your help, the simple removal of the <>'s fixed the problem. I'm new enough to not totally follow why filehandles caused the problem, yet the code worked on the first run through (the error version with filehandles included) and not on subsequent iterations, but I'm happy it works.

Also, I added the "use strict" and got a trillion warnings, so removed it to verify completion which it did, I'll work on making it strict friendly too.

Final question if someone has an understanding. This doesn't adjust my calculations at all, but is interesting for me in regards to better understanding. The first time through the for loop ($k=0), the 'print "$FN$k\n"' line prints the correct name (out-02-00.txt), but from then ($k>0) on I get an error that says "print() closed on filehandle OUT at isotopes-p7.perl line 14", which is the print line I just referred to. Is this because I "close(OUT)" at the bottom of the for loop and this is affecting the next iterations print statement until a new "OUT" file is opened? Any way to circumvent this? (I would just move the open(OUT) statement up, but I need $filename to correctly name it). Can I open a dummy out file for the time being or is there specific nomenclature for opening to terminal? I'll quit hazarding guesses and wait for an answer. Thanks again, any thoughts are welcome.