I didn't think that map{} could modify an input, but evidently that is true when the $_ var is modified.

It's clearly documented: "Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST." Just like foreach loops.

Many people use map { s/// }, but that clobbers the input in the process. List::MoreUtils's apply and Algorithm::Loops's Filter solve this problem, and so does

s/// for my @out = @in;
[download]

but one thing is that ("a","b","c","d") is different than ('a'..'d').

Indeed. A constant range in list context is flattened into an array.

$ perl -MO=Concise,-exec -e'print 1..3'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v
3  <0> pushmark s
4  <$> const[AV ] s       <-----
5  <1> rv2av lKP/1
6  <@> print vK
7  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

As previously mentioned, the problem is that the members of the array aren't read-only.