use strict;
use warnings;

my %h = (1=>2, 3=>4);

sub say { print @_, $/ };

sub get_keys {
    print 'called:';
    if (wantarray()) {
        print 'array:';
    } else {
        print defined wantarray() ? 'scalar' : 'void';
    }
    return keys %h;
}

                             # Output:
say sort get_keys;           # get_keys
say sort get_keys();         # 
say sort &get_keys;          # called:array:13
say sort &get_keys();        # called:array:13
say sort @{[get_keys]};      # called:array:13
say sort @{[get_keys()]};    # called:array:13