Your array contains but a single element, and that element is an anymonous array (created with square brackets --- perhaps that is a typo when posting your code?). Use parentheses for ordinary list construction and remember that sort() does not sort in-place but instead returns the sorted list:

my @array = ( -15, 991, -9, 980, 1, 911, 281, 1299, 870, 1205, 1, 1 );
@array = sort { $a <=> $b } @array;
print "@array\n";
__END__

-15 -9 1 1 1 281 870 911 980 991 1205 1299
[download]