As a side note, if you want to support returning false values (such as zero, but particularly undef) easily, return an empty list when the iterator is exhausted.

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

sub upto {
   my ( $m, $n ) = @_;
   return sub { $m <= $n ? $m++ : () };
}
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Then, use a list assignment in scalar/boolean context.

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

while ( my ( $ele ) = $iter->() ) { 
   say $ele;
}
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-2
-1
0
1
2
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You could have used while ( defined( my $i = $iter->() ). But you won't be able to use that for every iterators, such as one that returns the elements of an array:

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]";
}
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1
0
[undef]
a
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Since you sometimes need to use the empty-list "trick", you might as well always use it for consistency. Also, it's shorter than using defined :)