Modern readers can easily hear "evil eye" and think of a curse or superstition. In biblical idiom, however, an evil eye can describe the disposition of a person who looks begrudgingly upon another, especially in matters of generosity, possessions, and envy.
This background is particularly useful when Jesus uses eye language in contexts involving treasure, generosity, and resentment at another person's good fortune.
Eye language could function idiomatically as a description of character. A "good" or generous eye stands in contrast with an "evil" or begrudging eye. This usage helps connect the eye with attitudes toward possessions and toward the good received by another person.
The expression should therefore not automatically be imported into later folk beliefs about an "evil eye" curse. The Gospel contexts themselves point toward moral perception, envy, generosity, and divided loyalties.
Recognizing the idiom changes the question a reader asks. Instead of wondering about a mysterious power in the eye, the reader is directed toward the heart revealed by the way a person sees wealth and the good of others.
The ancient expression consequently sharpens Jesus' teaching about generosity and envy: the eye becomes a window into whether a person receives God's gifts gratefully or looks upon another person's good with resentment.