Interesting ...

It appears like read-only values are not modified when aliased.

  DB<123> sub tst { my $r = \ $_[0]; print ++$$r; }
 
  DB<124> $x=3; for (1,2,$x) { tst($_); print ": ",$_,"\n" }
2: 1
3: 2
4: 4
[download]

UPDATE:

Seems to me like "aliasing" read-only values is falling back to "copying".

Or in other words: Only lvalues can be "really" aliased.

Which makes sense in a way, because the original value can't be changed anyway.