chomp for @blocked; # don't expect to match newlines
if ( grep { $FORM{'comments'} =~ /\b$_\b/ } @blocked) {
  ...
}

##</code><code>##

chomp for @blocked;
for my $m (@blocked) {
  if ($FORM{'comments'} =~ /\b$m\b/) {
    # print your message here.
    exit;  #  could also last outside the loop
  }
}