use strict;
use warnings;
use 5.010;

my $child_pid = fork;
$| = 1;

if (!defined $child_pid) {
    say "**My error**: couldn't fork: $!";
}

if ($child_pid) { #then in parent
    say "in parent...";
    sleep 1;
}
else { #then in child
    open my $OUTFILE, '>', 'data1.txt'
        or die "**My error: couldn't open data1.txt: $!";

    my $old = select $OUTFILE;
    $| = 1;
    select $old;
    

    for (1 .. 10) {
        say $OUTFILE $_;
        sleep 2;
    }

    close $OUTFILE;
}


##</code><code>##

--output:--
$ perl 1perl.pl 
in parent...
$ cat data1.txt
1
2
$