is there a tutorial to learn how to translate this 'foreach / if' combination into 'map / grep'?

Effective Perl Programming item 20, "Use foreach, map and grep as appropriate", provides a nice 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