Try something like this (you may have to diddle the modifiers to s/// a touch):

my @data = <DATA>;

my @swap = qw/nce nse/;

my %replace = ( @swap, reverse @swap );

my $search_for = join('|', keys %replace);

s/($search_for)/$replace{lc $1}/gi foreach @data;

__DATA__
incense
license
luminance
[download]

update: added a /g to the s///