# @hash{map lc, @k} =
#   map lc, delete @hash{@k = keys %hash};

@keys = keys %hash;
@lc_values = map { lc $_ } @hash{@keys};  # hash slice
@lc_keys   = map { lc $_ } @keys;

delete @hash{@keys};  # basically:  %hash = ()
@hash{@lc_keys} = @lc_values;