Perl is only pass-by-reference by the way, sometimes it helps to see a
pass-by-reference example. So in Perl you can do this:

   sub some_function { $_[0] = 5 }

   my $a = 7;
   some_function($a);
   # now $a is 5

Perl has references, but the "reference" in "pass-by-reference" is a
different word, it means the parameters of the function are *aliases*
of the arguments in the caller.