> P.S. This is also sure to upset the eval haters :)

Trouble is rather that you might end with a bug depending on input if you try increment a hashref.

I would prefer

  DB<41> $ref= $tree

  DB<42> x @keys = split /@/,'a@b@c'
0  'a'
1  'b'
2  'c'
  DB<43> $ref = ($ref->{$_}//={})for @keys

  DB<44> x $tree
0  HASH(0xaa630cf0)
   'a' => HASH(0xaa6204a0)
      'b' => HASH(0xaa6719b0)
         'c' => HASH(0xaa75baa0)
              empty hash
  DB<45>
[download]

Update

And so on for each line

DB<45> x @keys = split /@/,'a@b@d'
0  'a'
1  'b'
2  'd'
  DB<46> $ref= $tree

<= ($ref->{$_}//={})for @keys

  DB<48> x $tree
0  HASH(0xaa630cf0)
   'a' => HASH(0xaa6204a0)
      'b' => HASH(0xaa6719b0)
         'c' => HASH(0xaa75baa0)
              empty hash
         'd' => HASH(0xaa672e30)
              empty hash
  DB<49>
[download]