文字列じゃなくちゃんと数値になってるよ。

use YAML;
use Scalar::Util;

print "\$YAML::VERSION = $YAML::VERSION\n";

my $arrayref = Load(<<'__HERE__');
- hoge
- 123
- true
__HERE__

foreach (@{$arrayref}) {
    # cf. http://blog.livedoor.jp/dankogai/archives/50916183.html
    print "$_: ", Scalar::Util::looks_like_number $_ ? 'num' : 'string', "\n";
}