sub from (&$) {
    use Devel::Callsite qw< callsite context >;
    use Scope::Upper    qw< reap UP >;

    my ($iterator_source) = @_;

    # Uniquely identify this call, according to its lexical context...
    my $ID = callsite() . context();

    # Install the iterator...
    state %iterator_for;
    $iterator_for{$ID} //= $iterator_source->();

    # Install destructor for iterator at end of caller's block...
    reap { delete $iterator_for{$ID} } UP UP;

    # Iterate...
    return $iterator_for{$ID}->($_[1]);
}


for my $limit (reverse 1..5) {
    while ( from{countdown($limit)} my $a ) {
        print "$a: ";
    }
    print "\n";
}

sub countdown{
    my $val = shift;

    my $iter = sub {
        if ($val--) {
            $_[0]=$val;
            return 1;
        }
        return;                         # stop iteration
    };

    return $iter;

}