The problematic line is this:

$self->{found_files} is a scalar and thus provides scalar context to the right side. So @found_files evaluates to the number of its elements, but you'll later treat it like an array reference. Just put a backslash in front of it to take its reference. The array is a copy of the arguments already so saying ... = [ @found_files ]; would do too but do extra needless copying.