It seems that
Ten percent of the pie is eaten
is now universally? considered correct. Was there a time in the history of modern English when this was not so? That is, when are would have been preferred?
For the record, I think a reasonable sentence analysis is the following.
- is/are : verb
- ten percent : subject (plural)
- of the pie : modifier of percent, which is subordinate to it
The current practice, I take it, is a contructio ad sensum (synesis), like when one says mathematics is fun, that's good news, or a means to an end. The verb form is determined by the ''logical'' subject (the pie-piece) rather then the grammatical subject (ten percent).