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