#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11129359
use warnings;
use Path::Tiny;

my $path = path( 'test3.txt' );

$path->spew(<<END);
hello world
foo_ foo
abc def
END

$path->edit_lines( sub { s/^(?=(foo_))/join '', map "$1$_\n", 1..3/e }
+);

print $path->slurp;
[download]

Outputs:

hello world
foo_1
foo_2
foo_3
foo_ foo
abc def
[download]