Solution with Iterators

        sub make_iterator {

          my ($file, $key) = @_;
          my $fh = do { local *FH }; 
          open $fh, $file or return;

          my $iterator =  
            sub {
              my $line;
              while ($line = <$fh>) {
                return $line if index $line, $key >= 0;
              }
              return;
            };
          return $iterator;
        }

Anonymous subroutine is a closure
my variables are captured by the closure
Each call to make_iterator constructs a new closure with new private state variables