#!/usr/bin/perl

use strict;
use warnings;

my $file = '/path/to/src.file';
my $dir = '/path/to/destination_directory/';

my $number_of_entries = 20; # Anzahl Einträge pro Datei;
my $counter = 0;

{
  local $/ = "\nDATA";
  my @entries = ();
  open(my $fh,"<",$file) or die $!;
  while(my $entry = <$fh>){
    chomp $entry;
    $entry = 'DATA' . $entry unless($entry =~ /^DATA/);
    push(@entries,$entry);
    if(scalar(@entries) == $number_of_entries){
      open(my $w_fh,">",$dir.$counter.".txt") or die $!;
      print $w_fh $_,"\n" for(@entries);
      close $w_fh;
      ++$counter;
    }
  }
  close $fh;
}

open(my $w_fh,">",$dir.$counter.".txt") or die $!;
print $w_fh $_,"\n" for(@entries);
close $w_fh;