Replace @-delimited string with expanded environment variable

The perl one-liner below will replace every occurrence of @some_string@ with the value of the environment variable some_string (i.e. what you get when you echo ${some_string}). For example, suppose you've got a file that contains the following text:

I like to eat @FRUIT@ @WHEN@.

If you run the one-liner on this file, with the environment variables FRUIT=apples and WHEN=daily, then the output will be:

I like to eat apples daily.