$ cat kidseek
#!/usr/bin/perl -w
use strict;

$| = 1;
print "One\nTwo\nThree\n";

if( ! fork() ) {
    seek( STDOUT, 4, 0 )
        or  die "Can't seek STDOUT in child: $!\n";
    print "Kid";
    seek( STDOUT, 0, 0 )
        or  die "Can't again seek STDOUT in child: $!\n";
    exit;
}

wait();

print "Dad";
 $ ./kidseek >kidseek.txt
 $ cat kidseek.txt
Dad
Kid
Three
 $