hello
world
foo_
foo
abc
def

##</code><code>##

use warnings;
use strict;

my $filename = 'test3.txt';

open(FH, '+<', $filename) or die $!;



while (my $line = <FH>) {

    if (my ($intro) = $line =~ m{ \A (foo_) }xms) {

      for my $x (1..3) {
        print FH "$intro$x\n"
       
        }
      }
    print FH $line;

    }

##</code><code>##

hello
hello
foo_
foo_1
foo_2
foo_3
foo_