sub update_db {
    # ...
}

sub mung_logline {
    # ...
}

pipe my($rd, $wr);

my $cpid;
{
    $cpid = fork;
    die $! if not defined $cpid;
    unless ($cpid) {
        close $wr;
        my $dbh = connect(,,,);
        while (<$rd>) {
            update_db( $dbh, $_);
        }
        close $rd;
        exit 0;
    }
}
close $rd;
{
    open my $fh, '<', '/path/to/log.file' or die $!;
    while (<$fh>) {
        $_ = mung_logline($_);
        print $wr $_;
    }
    close $fh;
}
close $wr;