use strict;

my %HoH;

# case 1:
{
    my $uid = 666;
    my $host = "foo.bar";
    $HoH{$uid}{$host} = "hold onto this value";
}
     # what I meant there was: please autovivify $HoH{$uid}
     # and set its value to be an anon.hash ref

# case 2:
{
    my $uid = "something_unexpected_or_possibly_undef";
    my $host = "who_cares_what_value_is_assigned_if_any";

    if ( exists( $HoH{$uid}{$host} )) {
         do_something_appropriate();
    }
}
    # what I meant there was: please do not autovivify $HoH{$uid}; instead,
    # do a separate check of hash-key existence for each level of hash nesting

##</code><code>##

use strict;
use warnings;

my %HoH;
if ( Exists( \%HoH, "foo", "bar", "baz" )) {
    warn "Something is very strange\n";
}
print scalar keys %HoH, "\n";

$HoH{foo} = { bar => { baz => "okay" }};
if ( Exists( \%HoH, "foo", "bar", "baz" ) and !Exists( \%HoH, "oops", "uhoh" )) {
    print "All is well\n";
}
print scalar keys %HoH, "\n";

# the following sub would actually be in a module:

sub Exists
{
    my  ( $href, $topkey, @subkeys ) = @_;
    return unless ( ref( $href ) eq 'HASH' );

    my $result = 0;
    if ( exists( $$href{$topkey} )) {
        $result = ( ref( $$href{$topkey} ) eq 'HASH' and @subkeys ) ?  
            Exists( $$href{$topkey}, @subkeys ) : 1;
    }
    return $result;
}