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