use strict;

sub to_case
{
    my ($char) = @_;

    if ($char eq "\U$char") {
        return sub {uc(shift)};
    } else {
        return sub {lc(shift)};
    }
}

print to_case("FOO")->("bAr") ."\n";
print to_case("foo")->("BaR") ."\n";

##</code><code>##

BAR
bar