I really didn't expect that much answers past one week of reaserch on that topic and finding not to much, THANKS!

your link really helped and brought me to the conclusion that it is very likely that the perl usemymalloc switch which is set on the server is causing the problem.

Unfortunately the sysadmin won't recompile perl without the switch (he will only use prepacked packages provided by HP)

Probably he will do an upgrade to 5.8 but this can take time... and (after reading the postings in your link) this will not fix the problem!

other answers: *) I can't map this to harddisk (performance issue)
*) I know 950MB is a lot of RAM but I will have to build up a complex Structure made up of small strings. This structure must be FAST handable. So that is why the RAM gets big!
*) And normally RAM is not the problem (why would someone by a server with 8CPUs and 16GB RAM if it's not for performance..., Just bad that perl can't keep scope with that!
*) the 4GB border is not the problem my problem would just need 10% more memory to handle the largest file, but by now all possible tweaks are already done (as I found them :-)

conclusion:
+++++++++++
I will have to rewrite the program and make it slower, but this seems to be less days of work then recompiling perl and checking all other scripts :-((((((

But anyway thanks for your response!