#!/usr/bin/perl -w
use strict;

my @array = ( 2, 4, 5, 5, 7, 8, 21, 22, 22, 30 );
my %count;

foreach ( @array ) {
  $count{$_}++;
}

foreach ( @array ) {
  print "$_ " if $count{$_} < 2;
}

Output:

2 4 7 8 21 30
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