I think that the confusion is my fault. You are correct that () = is forcing the matches of the regular expression to fill an anyonomous list (which should get optimized out), and are also correct that the match is being evaluated against the first element of @_ (in this case, the first argument to a subroutine). However, there is no magic being done on @_. The enclosing parens could have been completely left out. Here is my code fleshed out a little more:

sub expand_number_list {
    my @matches;
    () = ($_[0] =~ /
        (match something)
        (??{munge the match and push it to @matches})
    /xg);
    return @matches;
}
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Generally, the point is that just adding () = will force your expression into list context. Please see Forcing list context for more details.