sub upto {
   my ( $m, $n ) = @_;
   return sub { 
      return if $m > $n;
      return $m++;
   };
}

##</code><code>##

sub upto {
   my ( $m, $n ) = @_;
   return sub { $m <= $n ? $m++ : () };
}

##</code><code>##

my $iter = upto( -2, 2 );

while ( my ( $ele ) = $iter->() ) { 
   say $ele;
}

##</code><code>##

-2
-1
0
1
2

##</code><code>##

sub list {
   my @a = @_;
   return sub { @a ? shift( @a ) : () };
}

my $iter = list( 1, 0, undef, "a" );

# This doesn't work:
# while (  defined( my $ele = $iter->() ) ) {
#    say $ele // "[undef]";
# }

while ( my ( $ele ) = $iter->() ) ) { 
   say $ele // "[undef]";
}

##</code><code>##

1
0
[undef]
a