The key difference between the two (in this case, at least) is the conditional expression. A plain for loop will iterate faster than grep - but insert a conditional into the for loop, grep will win. Side note, in this thread I learned that grep in scalar context doesn't build the list, it just "returns the number of times the expression was true."^*

^* ripped directly from perldoc -f grep

And by the way, if you saw how my apartment was decorated before I got married, you'd never worry about my aesthetic sensibilities again ;)