my (%foo, %bar);
# Populate %foo and %bar

# Keys present in only one hash
my @only_in_foo = grep { ! exists $bar{$_} } keys %foo;
my @only_in_bar = grep { ! exists $foo{$_} } keys %bar;

# Matching keys which have differences in values
my @kv_different = map {
   if (exists $bar{$_} && $bar{$_} ne $foo{$_}) { $_ }
   else { () }
} keys %foo;

# Matching keys and values
my @kv_equal = map {
   if (exists $bar{$_} && $bar{$_} eq $foo{$_}) { $_ }
   else { () }
} keys %foo;