The issue is more about appropriate tools for the job than hammering various shaped pegs into whatever hole you have.

• use grep to get an output list that is a subset of an input list
• use map to transform an input list into an output list
• use for to iterate over a list (and almost never use the C version of for)
• use a hash to look up keyed data quickly
• use an array to lookup indexed data quickly
• because hash keys are unique a hash is a good tool for finding unique strings in a bag of strings