The stop filter
# isa Plucene::Analysis::TokenFilter
my $next = $stop_filter->next;
This removes stop words from a token stream.
Instances of the StopFilter class are tokens filters that removes from the indexed text words of your choice. Typically this is used to filter out common words ('the', 'a' 'if' etc) that increase the overhead but add no value during searches.
my $next = $stop_filter->next;
This returns the next input token whose term is not a stop word.