Here's a simple way to take the difference of sets.

use Data::Dumper;
%one = ( 'a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5, 'f' => 6,);
%two = ( 'a' => -1, 'e' => -2, 'i' => -3, 'o' => -4, 'u' => -5,);
print Dumper(\%one, \%two);

%tmp = %one;
delete @tmp{keys %two};
print Dumper(\%tmp);

%tmp = %two;
delete @tmp{keys %one};
print Dumper(\%tmp);
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Running the above produces the following:

$VAR1 = {
          'e' => 5,
          'c' => 3,
          'a' => 1,
          'b' => 2,
          'd' => 4,
          'f' => 6
        };
$VAR2 = {
          'e' => -2,
          'u' => -5,
          'a' => -1,
          'o' => -4,
          'i' => -3
        };
$VAR1 = {
          'c' => 3,
          'b' => 2,
          'd' => 4,
          'f' => 6
        };
$VAR1 = {
          'u' => -5,
          'i' => -3,
          'o' => -4
        };
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Ditto! However, instead of comm, I tend to use uniq in a pipeline.

(Slacking off and just doing the easier case for user id in the first field as opposed to the harder case of name extraction from the fifth field.)

Translating this to perl comes up with something like what's given in the following answers, but I find it's handy to test the rough idea on the command line if possible.

(EDIT: Removed the accidental dot between the paragraph and code tags. Moral: don't do markup after midnight.)