while (<$REIN>) {
    my $out_file_name = "out_file_nr_[$.]"; # $. is the line number on the last read file handle
    open my $RAUS, ">", $out_file_name or die "could not open $out_file_name $!";
    print $RAUS $_; 
    close $RAUS;
} 
close(REIN)