:P

if ( grep fc $str eq fc $_, keys %data ) { # case-insensitive exists
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Or to "fix" the data-

$data{fc$_} = delete $data{$_} for keys %data;
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As this example should make clear, and haukex's links explained, the entire operation is probably a bad idea as it is destructive (or at least has the appearance of being so if the plain folding is done without deleting.