The later edition of Effective Perl Programming has item 20, "Use foreach, map and grep as appropriate", which gives an excellent summary of when to use foreach, map and grep:

• Use foreach to iterate read-only over each element of a list
• Use map to create a list based on the contents of another list
• Use foreach to modify elements of a list
• Use grep to select elements in a list

Hall, McAdams and foy further caution against modifying a list via map:

"For efficiency, $_ is actually an alias for the current element in the iteration. If you modify $_ within the transform expression of a map, you modify the input data. This is generally considered to be bad style, and -- who knows? -- you may even wind up confusing yourself this way. If you want to modify the contents of a list, use foreach."

I'm pretty sure the earlier edition you are reading has a similar item. If you read this item, you should not be confused about which one to use. I suggest you focus on clarity and only bother benchmarking if performance is really critical.

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