Why did you use state there?

The problem is, it means that you cannot construct two concurrent iterators that work correctly.

With the line commented out in the following:

#my $_10_15 = upto_list( 10, 15 );
my $_5_30 = upto_list( 5, 30 );

show( $_5_30, 20 );
[download]

the show() line produces the expected 20 values:

C:\test>1078236.pl
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
[download]

But uncomment the 10/15 constructor and the show() now only produces 11 values despite requesting 20:

C:\test>1078236.pl
5
6
7
8
9
10
11
12
13
14
15
[download]