Because your first hash only contains one key, HASH(0x100804ed0). Had you used warnings, Perl would have told you so:

>perl -we "my %keys={A=>'b'};my ($a,$b); while (($a,$b)=each %keys) { 
+print qq($a $b\n); }"
Reference found where even-sized list expected at -e line 1.
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A hash is assigned a list, not a hash reference.