If I've understood your specification correctly then this should perform the tests you require.

use strict;
use warnings;

use Test::More tests => 2;

my $valid_in = 'a,b,c,d,e,f';
my $invalid_in = 'a,b,c,d,,f';

ok (test_it ($valid_in), "'$valid_in' is a valid argument");
ok (!test_it ($invalid_in), "'$invalid_in' is an invalid argument");

sub test_it {
    my ($string) = @_;
    my @fields = split (/,/, $string);
    my $emptyfields = grep { $_ eq '' } @fields;
    if (scalar @fields == 6 &&
        $emptyfields == 0) {
        return @fields;
    }
    return 0;
}
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