Your immediate problem is that x doesn't do what you want. To achieve what you want ("doubling" the commands) you need something like:

    push @array, ('@', '@') if scalar(@array) < 10;
    splice @array, -2, 2 if scalar(@array) >= 10;
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However that is only a tiny part of the problem. Your bigest problem is that an array is not appropriate, you need to use a string. Something like this:

use strict;
use warnings;
use Text::Autoformat;

my $str=qw/@/;
for my $i(1..10){
    $str = autoformat ($str, { justify => 'center' });
    print "$str\n";
    $str .= "@@" if length ($str) < 10;
    substr $str, -2, 2, '' if length ($str) > 10;
}
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However that still fails because you clobber $str each time through the loop. Changing the first two lines in the loop to :

    my $newStr = autoformat ($str, { justify => 'center' });
    print "$newStr\n";
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gets you closer, but leaves another logic problem (bug) which I'll leave you to puzzle over. :)