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

{   package Array::Nonempty::Attr;
    use Attribute::Handlers;

    sub Nonempty :ATTR(ARRAY) {
        my $referer = $_[2];
        tie @$referer, 'Array::Nonempty', sub { untie @$referer }
    }

}


{   package Array::Nonempty;
    use Tie::Array;
    use parent -norequire => 'Tie::StdArray';
    use Carp;

    sub CLEAR {
        my $self = shift;
        $self->[0] = [];
    }

    sub TIEARRAY {
        my ($class, $untie) = @_;
        bless [ [], $untie ], $class
    }

    sub EXTEND {
        my ($self, $size) = @_;
        croak "Cannot be empty" if 0 == @{ $self->[0] } && 0 == $size;
        $self->SUPER::EXTEND($size);

        # Prevent "untie attempted while 1 inner references still exist"
        my $untie = $self->[1];
        undef $self;

        $untie->()
    }

}

use parent -norequire => 'Array::Nonempty::Attr';

my %hash_ok = ( answer => 42 );
my %hash_empty = ();

my @keys_ok :Nonempty = keys %hash_ok;
say tied(@keys_ok) // 'not tied';
say for @keys_ok;

my @keys_empty :Nonempty = keys %hash_empty;