It doesn't work if there was an undef explicitly assigned to the value before. A lot of people get tripped up by this because they just assign undef to something they don't use anymore instead of deleting the key.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re^4: multiple values for one key in hash by dragonchild
in thread multiple values for one key in hash by sovixi

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