You're inadvertently creating a symbolic reference by using the string 'k1' as a hash. Using the 'V' option under the perl debugger you'll see something like the following (Note: I removed the my declaration so the original hash would show up in the symbol table) :

%hash = (
   'key1' => 'k1'
)
......
......
%k1 = (
   'key2' => 'k2'
)
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Just one more reason to use strict :) Note as well that it doesn't matter if it's a hash or hashref.