Calling the method set_fields should already assigns $self->{fields}, so it is redundant to
reassign it by calling the method.
Moreover, it could be dangerous in case the method set_fields do not contain as the last instruction
the assignment of $self->{fields}, as this will be assigned to whatever the last expression of
set_fields is evaluated to.
Removed redeclaration of lexically scoped variable.
Build the index of the format specifier fields.