#!/usr/bin/perl

use strict;
use warnings;

open my $fh, '<', '/pfad/zur/datei' or die $!;
local $/ = "\nproduct/productId";
while ( my $block = <$fh> ) {
    my $file = '/pfad/zur/ausgabe/' . $. . '.txt';
    chomp $block;

    open my $out, '>', $file or die $!;
    print $out "product/productId" if $. != 1;
    print $out $block;
    close $out;
}
close $fh;