# sub_program
use strict;
use warnings;

{ my $fh = select STDOUT; # making STDOUT "hot" (no IO buffering)
	  $| = 1;
	  select $fh;
}

for my $i (0..100) {
     print "$i \n";
     sleep 3;
}

##</code><code>##

# main program
use strict;
use warnings;

system ("perl sub_program.pl > file.txt &");

my $last_line;
for my $i (0..100) {
     sleep 10;
     local @ARGV = ("file.txt");
     $last_line = $_ while <>;
     print "iteration $i : $last_line";
}

##</code><code>##

$ perl  test_sub.pl
iteration 0 : 3
iteration 1 : 6
iteration 2 : 9
iteration 3 : 13
iteration 4 : 16
...