Observing a hash changes its contents...

I'm using a hash of hashes, and doing an existence check. If the check fails, I print a usage giving the valid keys. Strangely, the invalid key is being created by the existence check. I added a line to delete it, which fixes it, but why is this happening at all?

Here's a sample repro:

#!/usr/bin/perl -w

use strict;

my %hash = ('a' => {
                      'value' => 1,
                      'foo' => 'bar',
                   },
            'b' => {
                      'value' => 2,
                      'foo' => 'bar',
                   },
            'c' => {
                      'value' => 3,
                      'foo' => 'bar',
                   },
           );

if (! $hash{'d'}->{'value'}) {
        print "That key doesn't exist. Valid keys are: ";
        print join (", ", keys %hash);
}
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'd' appears in the output list. No change if I use defined() for the exist check.