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sub list_iter {
   my @list = @_;
...
while( my ($next) = $iter->() ) {
   print( $next // '[undef]', "\n" );
}

sub list_iter {
   my @list = @_;
...
while( $iter->( my $next ) ) {
   print( $next // '[undef]', "\n" );
}

a
1
...
0
[undef]
-1