map returns a new list, whereas for allows modification of an existing one

It sounds like contracdicting although, perhaps, you didn't mean it. The map operator allows modification as well since $_ would be a reference to the corresponding elements of existing array.

$ perl -wle 'my @ar = qw(one two three); print "@ar"; my @new = map { 
+$_ = uc } @ar; print "@new"; print "@ar"'
one two three
ONE TWO THREE
ONE TWO THREE
[download]