#!/usr/bin/perl
use warnings;
use strict;

open my $FH, '<', 'test.cfg' or die $!;
my $pos;
while (<$FH>) {
    $pos = $. if /echo.*install_list$/;       # Remember the line number.
}

die "Pattern not found.\n" unless $pos;

$. = 0;                                       # Reset the line number.
seek $FH, 0, 0;                               # Start reading from the beginning again.
open my $OUT, '>', 'test.cfg.new' or die $!;
while (<$FH>) {
    print {$OUT} $_;
    print {$OUT} "test\n" if $. == $pos;      # After the remembered line, print the extra text.
}
close $OUT;
rename 'test.cfg.new', 'test.cfg';