The Array::Diff SYNOPSIS hints that added and deleted return array references, not arrays:

use warnings;
use strict;
use Array::Diff;

my @old = ( 'a', 'b', 'c' );
my @new = ( 'b', 'c', 'd' );
    
my $diff = Array::Diff->diff( \@old, \@new );

my $cnt = $diff->count;
my $add = $diff->added;
my $del = $diff->deleted;
    
print "$cnt\n@{$add}\n@{$del}\n";

__END__

2
d
a
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