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

##</code><code>##

for ( keys %$ref) {
    ...
}

##</code><code>##

    $hist{$_}++ for keys %$_;

##</code><code>##

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