s/(nce|nse)/${{nce=>nse=>nse=>nce=>}}{$1}/g
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Or the perhaps simpler to understand:

%repl = (nce => 'nse', nse => 'nce');
foreach (...) {
   s/(nce|nse)/$repl{$1}/g;
}
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I see you have //i. If you want to preserve equivalent case in the replacements, you'll have to specify all the different combinations of case in the hash, or use s/(whatever)/mangle("$1")/ie where mangle is a sub that does whatever transformation you want.