use strict;

while ( <DATA> )
{
    my @line = split /\|/, $_;
    my @null = grep { defined $_ } map { $line[$_] =~ /^$/ ? $_ : undef } (0..$#line);
    print "line contains ", scalar @null, " null fields at offset(s): @null\n";
}

#line contains 3 null fields at offset(s): 1 3 4
#line contains 2 null fields at offset(s): 2 4
#line contains 3 null fields at offset(s): 0 1 4
#line contains 2 null fields at offset(s): 1 2 4

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