The words per, purr sound the same but have different meanings and spellings. Why do per, purr sound the same even though they are completely different words?
The answer is simple: per, purr are homophones of the English language.
To, for, or by each; for every: Gasoline once cost 40 cents per gallon.
According to; by: Changes were made to the manuscript per the author's instructions.
By means of; through.
Informal For each one; apiece: sold the cookies for one dollar per.
The soft vibrant sound made by a cat.
A sound similar to that made by a cat: the purr of an engine.
To make or utter a soft vibrant sound: The cat purred. The sewing machine purred.
To express by a soft vibrant sound.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.