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

Of course, that's only if you want to keep its case (mad props to Abigail-II for the nifty trick :-). However, if you want a more general mechanism for mapping particular words to other words and don't mind about the case:

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
[download]

That would be a more general mechanism even though it doesn't keep case.

Hope this helps.