The word unique nearly always call for a hash to be used. The solutiom is to extract the names, then to use them as (unique) keys to a hash.

#!/bin/perl -w
use strict;

my %unique_names;                          # this is the hash

while( <DATA>)
  { my @values= split /\|/;                # extract the fields (don't
+ forget to backslash the |)
    my @names= split /,\s*/, $values[2];   # extract the names
    foreach my $name (@names) 
      { $unique_names{$name}++; }          # we don't really care abou
+t the value, just the key
  }

my @unique_names= keys %unique_names;      # put the names back into a
+n array

print join "-", @unique_names;
print  "\n";

__DATA__
value 1|value 2|tom, chris, dave|value 4
value 1|value 2|peter, tom, dave|value 4
[download]