Just to expand slightly on pg's reply, perldoc perlop says

The "=>" operator is a synonym for the comma, but forces any word (consisting entirely of word characters) to its left to be interpreted as a string (as of 5.001). If the argument on the left is not a word, it is first interpreted as an expression, and then the string value of that is used.

And you would have gotten away with it, too, if it weren't for that meddling interpretation of non-words as expressions.