Given that undef is designed to do that job, I wonder why you would use anything else?

1. @a = (); clears the scalars and sets the notional size as reported by scalar @a or $#a to zero, but it does not release or reduce the basic AV. Thus it hangs on to some memory even though the array is 'empty';

   use Devel::Size qw[ total_size ];;
   @a = 1 .. 1e6;;
   print total_size( \@a );;
   32000176
   
   @a = ();;
   print total_size( \@a );;
   8000176
   [download]

2. splice @a; has the same, hang-on-to-some-of-it effect as @a = ();

   @a = 1 .. 1e6;;
   print total_size( \@a );;
   32000176
   
   splice @a;;
   print total_size( \@a );;
   8000176
   [download]

3. undef however, does a proper job and releases (almost) all the associated memory back to the process pool:

   @a = 1 .. 1e6;;
   print total_size( \@a );;
   32000176
   
   undef @a;;
   print total_size( \@a );;
   176
   [download]

   All it retains the essential head structure.