use strict;
use warning;

open IN, 'result.txt' or die "Error opening result.txt: $!";
   local $/ = undef;
   $_ = <IN>;
   close IN;

# this is the multi line text to be searched for
   my $pattern=" the patten is 
in many

line and @";

# after the match, there is some multi line text to be inserted, which I don't know how to.
        if(/\Q$pattern/){
              splice @array, $_ +1, 0, "inserting pattern in many

lines

with spaces
";
              
         }


   open OUT, '>', 'result.txt' or die "Error opening result.txt: $!";
   print OUT $_;
   close OUT;