perl cannot be written in Perl
If your statement is true, Perl is a less powerful language than say, BASIC or Cobol. See, perl is written in C, a Turing complete language. BASIC, Cobol, and almost any serious language (with the exception of some very domain specific languages) is Turing complete, which means that anything that can be done in one language, can be done in the other.
perl is written in C. Hence, perl can be written in BASIC and Cobol.
Luckely, your statement is false. Perl is a Turing complete language, and hence, perl can be written in Perl.