#!/usr/bin/perl

use strict;
use warnings;

my $input = 'file.txt';
my $output = 'out.txt';

open my $in, '<', $input or die $!;
open my $out, '>', $output or die $!;

while( my $line = <$in> ){
    if( $line =~ /Sachnummer: (\d+)/ ){
        print $out $1,"\n";
    }
}

close $out;
close $in;