my $pipe;

open($pipe, '-|', 'ip -o xfrm monitor') || die "couldn't start sub-command";

my $pid = fork();

if (! defined $pid) {
    die "couldn't fork";
elsif ($pid > 0) {
    # write $pid to .pid file
    cleanup();
    exit(0);                 ## problem happens HERE
}

## now in sub-process
close(STDIN);
close(STDOUT);
close(STDERR);
...

while (my $line = <$pipe>) {
   ...
}

close($pipe);
cleanup2();
exit(0);