the resulting listelements of a map are formed by the last command in the block, in your case it's the return value of s/// !

"Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string)."

Did you notice that you also manipulated @a by the way?

"Note that $_ is an alias to the list value" (from map)

...so @a holds the list you wanted 8 )

UPDATE: I'd rather preferre something like this  map { /(d)/ ? "new($1)" : $_ } @a