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# sub_program
use strict;
...
     print "$i \n";
     sleep 3;
}

# main program
use strict;
...
     $last_line = $_ while <>;
     print "iteration $i : $last_line";
}

$ perl  test_sub.pl
iteration 0 : 3
...
iteration 3 : 13
iteration 4 : 16
...