Question

Possible ways:

1. Using push:

   my @list;  
   push @list, 'foo' if $foo;  
   push @list, 'bar' if $bar;  
   

2. Using the conditional operator:

   my @list = (  
       $foo ? 'foo' : (),    
       $bar ? 'bar' : (),              
   );
   

3. Using the x!! Boolean list squash operator:

   my @list = (  
       ('foo') x!! $foo,  
       ('bar') x!! $bar,  
   );  
   

Which one is better and why?

Answer 1

Well, they all do different things.

However, all other factors being equal,

push @list, 'foo' if $foo;

is the statement that conveys its meaning most directly, so it should be preferred.

If you have to pause and think about statement that supposedly does something as simple as pushing an array element, you are doing something wrong.

my @list = (
    $foo ? 'foo' : (),
    $bar ? 'bar' : (),
);

could be OK if this is part of some colossal initialization that is being done elsewhere.

I think using

my @list = (
    ('foo') x!! $foo,
    ('bar') x!! $bar,
); 

indicates that the programmer has —how can I put this?— issues.

Incidentally, there is no such thing called the x!! composite operator. The !! is double logical negation. It converts an arbitrary false value to a defined but false value (the empty string) which yields 0 when used where perl expects a number. A true value is converted to a 1. Here, !! operates on $foo and $bar and writing it x!! $foo unnecessarily obfuscates the meaning of the code.

x is the repetition operator. So,

 ('foo') x !!$foo;

means repeat ('foo') once or not at all, depending on whether $foo is true or false, respectively.

PS: Of course, it turns out there was a PerlMonks article introducing the so-called boolean list squash operator. I read the article and find it unconvincing.