A variation on haukex's approach (assuming that guess about what you want is right):

>perl

use warnings;
use strict;

# use following statement (or similar) in real program:
# open my $fh, '<', 'actual_file_name' or die "opening: $!";
open my $fh, '<', \<<'EOD' or die "opening: $!";  # for demo only
hello
world
foo_
bar
foo
foox
baz
 foo_
foo_xyzzy
boff
EOD

LINE:
while (my $line = <$fh>) {

    if (my ($intro) = $line =~ m{ \A (foo) _ }xms) {
        print "$intro$_\n" for 1..3;
        next LINE;
        }

    print $line;

    }

^Z
hello
world
foo1
foo2
foo3
bar
foo
foox
baz
 foo_
foo1
foo2
foo3
boff
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