How do I create a file only if it doesn't exist?

You need to use the O_CREAT and O_EXCL flags from the Fcntl module in conjunction with sysopen: 

    use Fcntl;
    sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
		or die "can't open /tmp/somefile: $!":

Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS. That is, two processes might both successful create or unlink the same file!