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]";
}