my @mixedcasekeys = ('ABC', 'foo', 'ExAbbrev');
sub value { return "This is the value for $_\n"; }
my %hash;
for (@mixedcasekeys) {
    $hash{lc $_} = [$_, value($_)];
}
showkeys();
print "Adding key: 'Foo'\n";
$hash{lc 'Foo'} = ['Foo', value('Foo')];
showkeys();

print "Looping over the keys and showing the values:\n";
for $k (sort map {$hash{$_}[0]} keys %hash) {
   my $v = $hash{lc $k}[1];
   print "$k => $v\n";
}

sub showkeys {
  print "'Keys' in mixed case: " .
    (join ' ', map {$hash{$_}[0]} keys %hash) . "\n";
}