You forgot about join and map:

use strict;
use warnings;

while (<DATA>) {
     chomp;
     print join ' ', sort {$a<=> $b} map {/(\d+)-(\d+)/ ? $1 .. $2 : $
+_} split ',';
     print "\n";
}

__DATA__
18,5,1790,19-66,212,213
[download]

Prints:

5 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
+ 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
+ 64 65 66 212 213 1790
[download]