You could keep track of the project named in the last 'Rebuild All started' line and then use that for the next error.

my $project = '';
while (<BUILDLOG>) {
    if (/Project: (.+), Configuration/) {
        $project = $1;
    }
    if (/- (\d+) errors?, (\d+) warnings?/) {
        if ($1 > 0) {
            print "$1 errors for project $project\n";
        }
    }
}
[download]

As long as each project starts with a like like the 'Rebuild All started' line and projects are not interlaced in the file it should extract the correct project.