You probably meant

my $value = $it->();
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instead of

my $value->it();
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in the first sample.

As for the error: in order to use the block syntax with a (&)-prototyped subroutine, you need to declare the subroutine before you use it. Move the last line to the top of the second sample and everything should work.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

sub Iterator(&) { $_[0] }

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

my $it = upto(3,5);
while (my $value = $it->()) {
    say $value;
}
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