#!/usr/bin/perl -l
use strict;
use warnings;

while my ( $line = <DATA> ) {
  my $field1 = ( split /\./, $line, 2 )[0];
  print $field1;
}

__DATA__
Auto.VW.Golf.BJ
Auto2.VW.Polo.BJ
Auto3.Audi.A5.BJ
Auto4.Audi.A4.BJ
Auto5.Audi.A3.BJ