Iterator Example

This is a typical search-and-locate example (that is, it is a toy example)
Search a text file looking for matching lines
Obvious way:

        sub locate {
          my ($key, $file) = @_;
          local *FH;
          open FH, $file or return;
          my @result;
          while (<FH>) {
            push @result, $_ if index($_, $key) >= 0;
          }
          @result;
        }

Maybe we spent a lot of time locating all the matching lines
Maybe we only needed a couple
Maybe the file was very big
Maybe several different parts of the program need to look for different things