#! perl -sw
use strict;

my %foo = (
  aa => { O => 'zz',  C => 'yy'  },
  bb => { O => 'xx',  C => 'xx'  },
  cc => { O => undef, C => 'ww'  },
  dd => { O => 'vv',  C => undef },
  ee => { O => undef, C => undef },
  ff => {             C => undef },
  gg => { O => 0,     C => 0     },
  hh => { O => undef, C => 0     },
);

sub BUK{
  local($^W,$a)=(0,pop);
  grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a->{$_}{C}}keys %$a;
}

sub snw{
  no warnings qw(uninitialized);
  my$h=pop;
  grep{$h->{$_}{O}ne$h->{$_}{C}}keys %$h;
}

sub sg_{
  my$h=pop;
#        defined($x         &&$y)         &&$x         eq$y         
   grep{!(defined($h->{$_}{O}&&$h->{$_}{C})&&$h->{$_}{O}eq$h->{$_}{C}
# ||!defined($x         ||$y)
   ||!defined($h->{$_}{O}||$h->{$_}{C}))}%$h;
}

sub eg_{
  my$h=pop;
#       (($x         ||$;)eq($y         ||$;))
  grep{!(($h->{$_}{O}||$;)eq($h->{$_}{C}||$;))}%$h;
}
=pod
sub ag_{
  my$h=pop;
  grep sub{(1==grep defined,@_)or($_[0]ne$_[1])}
       ->(values @{$h->{$_}}['O','C']), keys %$h;
}
Type of arg 1 to values must be hash (not array slice) at 
C:\test\201391.pl line 26, near "])"
=cut

print "      Required: dd ff aa hh cc\n";
print "-------------------------------\n";
print "BrowserUK golf: @{[BUK(\%foo)]}\n";
print "sauoq No warns: @{[snw(\%foo)]}\n";
print "    Sauoq golf: @{[sg_(\%foo)]}\n";
print "  Elusion golf: @{[eg_(\%foo)]}\n";
#print "Aristotle golf: @{[ag_(\%foo)]}\n";

=pod
It seemed to me that the biggest problem with the original problem was the method of recording the old versus changed hash--by adding an extra key. 
Better I think to uses two hashes, %old and %change (or %o %o here). That makes the problem of noting the changes somewhat clearer I think. Though it does little better in the golf stakes.
=cut

my %o=(aa=>'zz', bb=>'xx', cc=>undef, dd=>'vv', ee=>undef, gg=>0, hh=>undef,);
my %c=(aa=>'yy', bb=>'xx', cc=>'ww', dd=>undef, ee=>undef, ff=>undef, gg=>0, hh=>0,);

sub comp{
    local($^W, $a, $b)=(0, @_);
    grep{!exists$a->{$_} or $a->{$_} ne $b->{$_} } keys%$b;
}

print "     BUK2 golf: @{[comp(\%o,\%c)]}\n";