Alright :)
Would this be correct. Looks like it works:

use strict;

my $pid = fork();
if($pid == 0)
{
        `perl write.pl f1.txt`;
        exit(0);
}
my $pid = fork();
if($pid == 0)
{
        `perl write.pl f2.txt`;
        exit(0);
}
exit(0);

### where write.pl is:

use strict;
my $file = $ARGV[0];
open OF, ">$file" or die "cannot open file $file\n";
for(1..10) {print OF time()."\n"; sleep 1}
exit(0);
[download]

The time printed in files overlaps.