That's the way references work - what you're passing to your 'pass1' function is a pointer to the hash '$hashref' points to, not a copy of it, so local changes get applied to that global copy. If you want to pass a local copy, specify that:

pass1({ params => {%$hashref} });  # {%$hashref} makes a (shallow) cop
+y of the hash '$hashref' 
                                   # points to and creates a new ref t
+o it.
[download]

If you do that, then you get:

before pass1: 1
Inside pass1...
2
3
after pass1: 1
[download]

which is what I think you expect.

Added (shallow) note, AnomalousMonk is correct in his reply below!