The idea with the habit of adding () after all sub-calls is indeed a very good idea.

There is a special case where the use of parentheses is not appropriate. If you call subB from within subA using the syntax &subB with no parentheses then the argument list of subA will be passed to subB. See this node for an example.

The subroutine doesn't actually have to be called from within another subroutine; it is the content of @_ that matters, as demonstrated by the following code.

$ perl -Mstrict -Mwarnings -E '
@_ = @ARGV;
∑

sub sum
{
    my $sum;
    $sum += $_ for @_;
    say $sum;
}' 1 2 3 4 5
15
$
[download]

I hope this is of interest.