In Regexes§
See primary documentation in context for Samecase
The :samecase
or :ii
substitution adverb implies the :ignorecase
adverb for the regex part of the substitution, and in addition carries the case information to the replacement string:
= 'The cat chases the dog';s:global:samecase[the] = 'a';say ; # OUTPUT: «A cat chases a dog»
Here you can see that the first replacement string a
got capitalized, because the first string of the matched string was also a capital letter.