jwkrahn's suggestion of a hash-of-hashes is probably best, but if you're married to an anonymous array for holding your users, you can uniq-ify (and maybe sort at the same time) each user array after all data is collected.

>perl -wMstrict -le
"use List::MoreUtils qw(uniq);
 use Data::Dumper;
 ;;
 my %table = (
   foo => [ qw( c b a    b a c )   ],
   bar => [ qw( g f e d  f g d e ) ],
   baz => [ qw( i h      h i)      ],
   );
 ;;
 $_ = [ sort { $a cmp $b } uniq @$_ ] for values %table;
 ;;
 print Dumper \%table;
"
$VAR1 = {
          'bar' => [
                     'd',
                     'e',
                     'f',
                     'g'
                   ],
          'baz' => [
                     'h',
                     'i'
                   ],
          'foo' => [
                     'a',
                     'b',
                     'c'
                   ]
        };
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