use Data::Dumper;
my $x=[1, , ,2];         # This is NOT the same as $y !!!!
my $y=[1,undef,undef,2];
print Dumper($x,$y);
__END__
$VAR1 = [
          1,
          2
        ];
$VAR2 = [
          1,
          undef,
          undef,
          2
        ];