The explicit passing of shell did help, thank you. You are absolutely right. I obviously did some mistake with the shell scripting before - I apologize. I re-attempted #!/bin/sh \\ ulimit -f ... and it does write a 2560 byte file on RHEL6. So it's not perl at all, but bash doing different things depending on $0 (/bin/sh does point to bash on both). So I'll fix the code to pass the shell explicitly to system().

Obvious now, but just to answer your question, ulimit does succeed on both.

Thank you once again!