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

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

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

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

-2
-1
0
1
2

sub list {
   my @a = @_;
...
while ( my ( $ele ) = $iter->() ) ) { 
   say $ele // "[undef]";
}

1
0
[undef]
a