my $line = ";field;;field;\t;field; ;field;field";
my @line = split ";", $line;
my @null = grep { defined $_ } map { $line[$_] =~ /^$/ ? $_ : undef } (0..$#line);

print "line contains ", scalar @null, " null fields at offset(s): @null";
#line contains 2 null fields at offset(s): 0 2