I'd create a translate key hash (called %xl8 in code example below).

 
%hash1 = ("John", 43, "Paul", 25, "Marie", 22);
%hash2 = ("john", 43, "Paul", 25, "marie", 22); 

foreach (keys %hash1) { $xl8{lc $_} = $_; }


while (($KEY_2, $VALUE_2) = each %hash2){
    if (exists $xl8{lc $KEY_2}){
         print "$KEY_2 : Matched\n";
    }
    else{
         print "$KEY_2 : Did not match\n";
    }
}
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(looks like I was too slow... someone else suggested same with the elegant "map" solution. :)