Well, "read-only file system" makes me think that the two cases are running the command with different working directories or with different privileges (or perhaps from different computers, but I would expect you to have told us that case). For example, if the Perl script is a CGI script...

If you are running the Perl script from a command line and the script doesn't do things like chdir() or open(STDOUT,...), then system("...") is nearly identical to running the command directly. But none of the differences make sense as causing that error (the process parent is perl instead of some shell, the process probably isn't the process group leader in one case and probably is in the other, ...).

        - tye (but my friends call me "Tye")