use strict;
my %hash = ();
$hash{a}{drinks}=1; 
$hash{b}{drinks}=2;
print keys %hash;  						 	  
## printed a b
print map { $hash{$_}{drinks}  } keys %hash;  
## printed 1 2 

my $p = \%hash;
my %copy = %{ $p };
print keys %copy;
## printed a b 
##   - fine, %hash has been copied to %copy
$copy{c}=3;
print keys %hash;                        	  
## printed a b 
##   - ok, %copy is something else 
##   and %hash won't change when %copy changes.

print "REF_HASH=", \%hash, ", REF_copy=", \%copy;  
## REF_HASH=HASH(0xcfbf0c), REF_COPY=HASH(0xd05168) 
##   - yup, they're different indeed.

$copy{a}{drinks}=4;
print map { $hash{$_}{drinks}  } keys %hash;  
## printed 4 2 !?!?!?! 
## I don't get it! 
##  Why did %hash change here but didn't before, 
##   when I added the new key 'c'?