foreach ($ref) {
    $hist{$_}++ for $_;
}
[download]

Your $ref is a hash reference - use keys:

for ( keys %$ref) {
    ...
}
[download]

Since your data structure is a HoH, inside that loop $_ is, again, a hash reference. Again you need keys.

    $hist{$_}++ for keys %$_;
[download]

But such usage of $_ is a bit obfuscated. Better say

for my $hashref ( keys %$ref) {
    $hist{$_}++ for keys %$hashref;
}
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}