If the problem is ensuring that your fake addresses are always unique, why not just append an incrementing number from within the script?

my $uniq = 0;
sub create_random {
   my @chars = ( "A" .. "Z", "a" .. "z", 0 .. 9, qw(! @ $ % ^ & *) );
   my $user_name = join("", @chars[ map { rand @chars } ( 1 .. 10 ) ])
+;
   return $user_name . $uniq++;
}
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If you need to keep the uniqueness across runs, store a single field for the number in the DB, query it when the script starts and store it before you exit.