# pseudo code that will not run unless you do some homework...
use strict;
use warnings;

my $big_var = "Some massive\nstring\n"; # that big 200 line var goes here
my $pattern = "<TARGET>";

open my $input  input file
open my $output output file

while read line from input {
    print to output
    if pattern is matched {
        print $output $big_var
    }
}

close both files
copy output over input