... provides an historical perspective on the promotion of English in India ...and the editor changed 'an historical perspective' to 'a historical perspective'.
The reason for this editorial change is presumably because there is a consonant at the start of historical, so the editor feels that a rather than an should be used.
However, we should note that, in actual speech, initial /h/ is often omitted from function words such as his and her because they are generally unstressed. And the main stress in historical is on the second syllable, so the initial /h/ in the unstressed first syllable is actually usually omitted. This means that historical is in fact often spoken with an initial vowel, so it is best to use an rather than a before it.
In contrast, history has its main stress on the first syllable, so the /h/ is not omitted, and we say 'a history of Brunei'.
OK, so this is very, very picky, and I allowed the editor to change the usage if he wishes; but maybe it is of interest to think a little about the rationale for the use of an or a. Note that we say 'an hour' not *'a hour' because the initial /h/ in hour is silent. Note also that we say 'a university', not *'an university', because university actually begins with the consonant /j/.