use strict; 
use warnings; 
use feature 'say';

my @x = ( -2, 3, 2, 10, 15, 8 );
my %y = map { $_ => 1 } @x;

@x = sort {$a <=> $b} @x;

say "missing: $_" for grep { ! exists $y{$_} } $x[0] .. $x[-1];