I think that was a fine solution to the problem. Why not use CGI.pm? And sure, there are many ways to tackle this problem - here is another that utilizes your ternary suggestion:

use strict;

my @ss = ('SSD','PERS','SS','CHILD SUP');
my %checked = map {$_ => 1} ('SSD','PERS');

foreach my $ss (@ss) {
   print qq|<input type="checkbox" name="ss" value="$ss"|,
      $checked{$ss} ? ' selected' : '', ">\n"
   ;
}
[download]

UPDATE:
Actually, @checked should be populated with something like the original:

my %checked = map {$_ => 1} split(/,/,$INPUT{'ss'})
[download]

But that point is moot (mu?) - use CGI.pm to handle form processing.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)