An alternative that feels cleaner, though (*MARK:NAME) is supported way fewer places than named captures:

$ perl -E'say "xxx" =~ s/(*MARK:a)^.|(*MARK:c).$|(*MARK:b)./$REGMARK/g
+r'
abc
[download]

$ python -c 'import pcre2; print(pcre2.sub("(*MARK:a)^.|(*MARK:c).$|(*
+MARK:b).", "$*MARK", "xxx"))'
abc
[download]