You might think of

$hash{a}{drinks}=1; 
$hash{b}{drinks}=2;
[download]

as a single hash, but that's not the case. There are three:

• %hash
• %{ $hash{a} }
• %{ $hash{b} }

+-------+     +----------+    +-----------+    +-------------+
| %hash | +-->| Anon Ref | +->| Anon hash | +->| Anon scalar |
+-------+ |   +----------+ |  +-----------+ |  +-------------+
| a   ----+   | addr   ----+  | drinks  ----+  | 1           |
| b   -----+  +----------+    +-----------+    +-------------+
+-------+  |
           |  +----------+    +-----------+    +-------------+
           +->| Anon Ref | +->| Anon hash | +->| Anon scalar |
              +----------+ |  +-----------+ |  +-------------+
              | addr   ----+  | drinks  ----+  | 2           |
              +----------+    +-----------+    +-------------+
[download]

When you do %copy = %hash (which you wrote as %copy = %{ \%hash }), you are copying the keys and values of %hash. So what are the keys and values of %hash?

>perl -e"$h{a}{d}=1; $h{b}{d}=2; print qq{$_: $h{$_}\n} for keys %h;"
a: HASH(0x22534c)
b: HASH(0x2253a0)
[download]

That's all that's copied, nothing more. It will not make a copy of the referenced value and make a new reference to it. It simply copies the references. You end up with

+-------+     +----------+         +-----------+    +-------------+
| %hash | +-->| Anon Ref |  +----->| Anon hash | +->| Anon scalar |
+-------+ |   +----------+  |      +-----------+ |  +-------------+
| a   ----+   | addr   -----+      | drinks  ----+  | 1           |
| b   -----+  +----------+  |      +-----------+    +-------------+
+-------+  |                |
           |  +----------+  |      +-----------+    +-------------+
           +->| Anon Ref |  |  +-->| Anon hash | +->| Anon scalar |
              +----------+  |  |   +-----------+ |  +-------------+
              | addr   --------+   | drinks  ----+  | 2           |
              +----------+  |  |   +-----------+    +-------------+
                            |  |
+-------+     +NEW-------+  |  |
| %copy | +-->| Anon Ref |  |  |
+-------+ |   +----------+  |  |
| a   ----+   | addr   -----+  |
| b   -----+  +----------+     |
+-------+  |                   |
           |  +NEW-------+     |
           +->| Anon Ref |     |
              +----------+     |
              | addr   --------+
              +----------+
[download]

What you want is to do is to copy referenced values as well, and to do recursively, so that you end up with the following:

+-------+     +----------+    +-----------+    +-------------+
| %hash | +-->| Anon Ref | +->| Anon hash | +->| Anon scalar |
+-------+ |   +----------+ |  +-----------+ |  +-------------+
| a   ----+   | addr   ----+  | drinks  ----+  | 1           |
| b   -----+  +----------+    +-----------+    +-------------+
+-------+  |
           |  +----------+    +-----------+    +-------------+
           +->| Anon Ref | +->| Anon hash | +->| Anon scalar |
              +----------+ |  +-----------+ |  +-------------+
              | addr   ----+  | drinks  ----+  | 2           |
              +----------+    +-----------+    +-------------+

+-------+     +NEW-------+    +NEW--------+    +NEW----------+
| %copy | +-->| Anon Ref | +->| Anon hash | +->| Anon scalar |
+-------+ |   +----------+ |  +-----------+ |  +-------------+
| a   ----+   | addr   ----+  | drinks  ----+  | 1           |
| b   -----+  +----------+    +-----------+    +-------------+
+-------+  |
           |  +NEW ------+    +NEW--------+    +NEW----------+
           +->| Anon Ref | +->| Anon hash | +->| Anon scalar |
              +----------+ |  +-----------+ |  +-------------+
              | addr   ----+  | drinks  ----+  | 2           |
              +----------+    +-----------+    +-------------+
[download]

That's called a "deep copy" because it copies every level of the data structure. In contrast, a simple assignment is called a "shallow copy" because it only copies the immediate value (scalar/hash/array/...).

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