It really falls under the category of one of those things that is possible but not all that advisable.

Just because it's possible to say, "Let's interpolate a function: @{[PI]}\n";, I'm not convinced that it's in any way clearer than: "Lets not interpolate: " . PI . "\n";

In the first example the double-quoted string is parsed for interpolation components. The @{...} is found, and that tells Perl you want to interpolate a reference to an array. And the [....] creates that reference, based on the "...." expression.

In the second example, you start with a string, concatenate the evaluated expression, and concatenate a newline character. Much simpler, and less work.