>perl -Mstrict -le
"my $s = 'foo,,';
 ;;
 my ($field0, $field1, $field2, $field3) = split /,/, $s;
 print qq{field0 '$field0'  field1 '$field1'  field3 '$field3'};
 ;;
 if ('bar' =~ m{ $field1 }xms) { print qq{bar matches '$field1'} }
 if ('bar' =~ m{ $field3 }xms) { print qq{bar matches '$field3'} }
"
field0 'foo'  field1 ''  field3 ''
bar matches ''
bar matches ''