if(  grep( $_ !~ /^\d+$/, @{$INPUT{'checked_ids'}) ){
    # you have bad data
  }

##</code><code>##

  my @ids = split (/, +/, $INPUT{'checked_ids'});
  # now check @ids w/grep() using the method above

##</code><code>##

  if( $INPUT{'checked_ids'} !~ /^(\d+(, +)?)+$/ ){
    # you have bad data
  }