update 2004/8/10: Red Hat has shown me (by invoking the perl in a 'kernel backward compatibility mode' that there is some (as yet unidentified) incompatibility between perl 5 and the new kernel, probably in the threading code. We're still trying to peel the onion. Thanks for your responses - a little feedback for the interested. The child process never gets to main() in the hang situation, (ie is not loooking for input) In fact, swapping 'ls' in as the called program doesn't change the behavior. I have a working perl program that, among other things, uses system() to run some other programs. I upgraded a Linux box from RH 9.3 to Enterprise Linux WS 3.1 and the program no longer works. (This is Perl 5.004_04) I can see that perl has forked, but the child is hanging, and the parent is waiting. ^C interrupts the child, and the parent completes. I discovered that if I run the program in `backticks`, it behaves properly. But I don't want to seek out all the system() calls & replace them & I'd like to know what is going on. So, I am interested in any insight about what is different enough about these two ways of running the program, that might make the difference. Or if you have heard of problems in this version of RH Linux. (It is not 'convenient' to upgrade the perl version, BTW) I'd be happy for any clues about where to look.