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my $joined_keys = 

...
#...............................^^^^^^^^^^^^^^^^^...^^^^^^^^^^^^^^
#...the combined (potentially huge) lists of the keys from both hashes
+ PLUS
#.....................^^^^^...undef as the value for each key in the c
+ombined list...

sub mergedHashKeys{ 
[ keys %{ {                           %{ $_[0] },         %{ $_[1] }  
+ } }  ] 
};;