Sure. That uses the "trinary operator", which has the form foo ? bar : baz. It tests foo, whatever that may be, for truth. If true the expression evaluates to bar, if false, to baz.

Here, foo corresponds to a regular expression match with $entry. The regex tests that $entry consists of exactly three digits and captures the three to $1, $2 and $3 by surrounding each digit's slot with parentheses.

The bar element is an explicit list of combinations using the captured digits, whatever they may be. It will be returned to the @array assignment by the expression if the match succeeded. The "baz" element is an empty list, which goes to the assignment if the match failed.

It is important to never use the backreference variables $1,$2,... unless the regex match succeeded. The trinary op above complies with that, since the number variables only appear in the true branch.

The trinary op is documented in perlop. ++bobf for suggesting the link.