#!/usr/bin/perl

use strict;
use warnings;

my %null;

while (<DATA>){
  my @fields = split(/\|/);
  for my $i (0..$#fields){
    $null{"field$i"}++ unless $fields[$i];
  }
}

for my $key (sort keys %null){
  print "$key->$null{$key}\n";
}

__DATA__
first||third||
alpha|beta||delta|
||c|d|
one|||four|