Qué would sound more life you're asking them to define their version of the word perspective. Cuál is used to ask quantitative things like "what number is it?" and "what day is it?" whereas qué is used for "what did you do?" and "what is this?" if that helps any. One thing NOT to do is to try directly translating them as what or which. That would certainly confuse more than help.
Just the way things are sometimes, e.g. 'Cual es su problema' - 'What is your problem'.
Here is a link with a pretty thorough explanation:
Why "what is THEIR perspective" is not accepted ?!? (Without knowing the context ...)
Because "their" is a plural pronoun, not a singular pronoun. "Their" means more than one person. "Their" can't be used as a him/her substitute in English or Spanish.