perl -MO=Deparse -e "$h{512_x64} = 10; print $h{512_x64}, qq/\n/;"


##</code><code>##

x</x> operator stringifies it, so that the number 512 is stringified and repeated 64 times.</p>
<p>In your definition (ie, <c>%h = ( 512_x64 => 'value' );