# first, scan the first file, noting the file pos's on which each key occurs.

	my %key_pos_in_first_file; # key=key, val=array of file positions.

	open F1, "+< $first_file" or die "open $first_file for random read - $!\n";
	my $p1 = 0;
	while (<F1>) {
		chomp;
		my @l = split /\|/;
		push @{ $key_pos_in_first_file{ $l[0] } }, $p1;
		$p1 = tell F1;
	}

	# second, go through the second file, joining.

	open F2, "< $second_file" or die "open $second_file for read - $!\n";
	while (<F2>) {
		chomp;
		my @l2 = split /\|/;
		# go to each pos in the first file and use that line
		for my $p1 ( @{ $key_pos_in_first_file{ $l2[0] } } ) {
			seek F1, $p1, 0;
			my $l1 = <F1>;
			chomp $l1;
			my @l1 = split /\|/, $l1;
			# join
			print "@l2 - @l1\n";
		}
	}
	close F2;
	close F1;