My tool of preference here is split. You could do something like this:

my @hosts;
while (<INPUT>) {
    # log file lines have multi-character domain names at the start
    if (/^\w+/) {
        my $file = (split(/\|/, $_, 3))[2];
        if ($file =~ s!/access_log\.(.+)$/) {
            push @hosts, $1;
        }
    }
}
[download]

Other approaches are equally valid. (Hey, davorg's book is full of nice tricks like this.)