Here's a simple script that will do what you're asking. Rather than editing the existing file, it redirects to a new file "text.out". If you need to, simply add a system command to rename text.out to basic.txt. Obviously, there are better/different ways of handling this, but you weren't very specific about your scenario.

-fuzzyping

#!/usr/bin/perl

use strict;

my $string_to_match = "I dont have money";
my $string_to_insert = "yes thats correct\nbut your should have\nmoney
+ with you\notherwise it is not\npossible\n";

open(INFILE, "basic.txt");
open(OUTFILE, ">>text.out");

while (<INFILE>) {
        unless (/$string_to_match/) {
                print OUTFILE $_;
        } else {
                print OUTFILE $_, $string_to_insert;
        }
}

close(OUTFILE);
close(INFILE);

1;
[download]