s/(n(?i:s|c)e)/$1 ^ "nse" ^ "nce"/gie;

##</code><code>##

my %word_map = (
  nce => "nse",
  nse => "nce"
);
my $re = join "|", map qr/\Q$_/i, keys %word_map;
s/($re)/$word_map{lc $1}/ge;
print