my $joined_keys = 

[ keys( %{ { map { $_=>undef }  keys( %{$hash1} ) , keys( %{$hash2} ) } } ) ];
# This.....^............................................................^ *is a hash* built from 
#...............................^^^^^^^^^^^^^^^^^...^^^^^^^^^^^^^^
#...the combined (potentially huge) lists of the keys from both hashes PLUS
#.....................^^^^^...undef as the value for each key in the combined list...