My initial thought for the capitalization is something like this:

s/(bad)/my $str = "sad"; if (substr($1, 0, 1) eq 'B') { ucfirst $str }; $str/gie;

but that strikes me as ugly. Another approach would be building a hash with the possibilities:

my %insens = (
    bad => 'sad',
    Bad => 'Sad',
    bAd => 'sAd',
);

s/(bad)/$insens{$1}/gi;
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Build the hash programmatically if necessary.