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    if (my $pid = fork) { # $pid defined and !=0 -->parent
        ++$forkcount;
...
        close $IN;
        defined $pid and exit(0); # $pid==0 -->child, must exit itself
    }

my $pid=fork() // die "Can't fork: $!";
if ($pid) {
...
} else {
    # child code
}

my $pid=fork();
defined($pid) or die "Can't fork: $!";