chester and svenXY have told you why it does not do what you want and how you can init the slice. Here is a quick way to init if you want to do it for a large number of indices.

use strict;
use warnings;
use Data::Dumper;


my @x;
@x[0..3] = map {1}0..3;  # this map returns a list of 1's (4 elements)

print Dumper(\@x);
__END__
$VAR1 = [
          1,
          1,
          1,
          1
        ];
[download]

BTW, you had @x[1..3] I changed it @x[0..3]. I felt that is probably what you want as the array index starts with zero. If you are sure you don't need zero then you can modify my code.

-SK