I read that PERL internally uses UTF-8 format.
Where did you read that? Certainly not from perlunitut which says (my emphasis):
Perl has an internal format, an encoding that it uses to encode text strings so it can store them in memory. All text strings are in this internal format. In fact, text strings are never in any other format!
You shouldn't worry about what this format is, because conversion is automatically done when you decode or encode.