##
use warnings;
use strict;
my $filename = 'test3.txt';
open(FH, '+<', $filename) or die $!;
while (my $line = ) {
if (my ($intro) = $line =~ m{ \A (foo_) }xms) {
for my $x (1..3) {
print FH "$intro$x\n"
}
}
print FH $line;
}
## ##
hello
hello
foo_
foo_1
foo_2
foo_3
foo_