Copy selected lines from one file and insert those lines in another file after selected line

Question

Here is my question let's say I have a file file1.txt with contents:

abc.. 
def.. 
ghi.. 
def..

and second file file2.txt with contents:

xxx..
yyy..
zzz..

Now I want to copy all the line starting with "def" in file1.txt to file2.txt and append after "yyy..." line in file2.txt

Expected Output:

xxx...
yyy...
def...
def...
zzz...

I am pretty much new to perl, I've tried writing simple code for this but end up with output only appending at the end of file

#!/usr/local/bin/perl -w 
use strict; 
use warnings;
use vars qw($filecontent $total);
my $file1 = "file1.txt";
open(FILE1, $file1) || die "couldn't open the file!";
open(FILE2, '>>file2.txt') || die "couldn't open the file!";
  while($filecontent = <FILE1>){
    if ( $filecontent =~ /def/ ) {
      chomp($filecontent);
       print FILE2 $filecontent ."\n";
      #print FILE2 $filecontent ."\n" if $filecontent =~/yyy/;
    }  
  } 
 close (FILE1); 
 close (FILE2);

output of the perl program is

xxx...
yyy...
zzz...
def...
def...

Answer 1

I'd use a temp file.

1. Read and print all lines (to temp) from FILE2 until you hit "yyy"
2. Read and print all "def" lines (to temp) from FILE1
3. Read and print (to temp) the rest of FILE2
4. Rename the temp file to FILE2

use strict; 
use warnings;

my $file1 = "file1.txt";
my $file2 = "file2.txt";
my $file3 = "file3.txt";

open(FILE1, '<', $file1) or die "couldn't open the file!";
open(FILE2, '<', $file2) or die "couldn't open the file!";
open(FILE3, '>', $file3) or die "couldn't open temp file";

while (<FILE2>) {
  print FILE3;
  if (/^yyy/) {
    last;
  }
}

while (<FILE1>) {
  if (/^def/) {
    print FILE3;
  }
}

while (<FILE2>) {
  print FILE3;
}

close (FILE1); 
close (FILE2);
close (FILE3);

rename($file3, $file2) or die "unable to rename temp file";