sub build_rec {
        my ($self, $middlex, ...) = @_;

        my $middlex /= 2;
        ...;

        if ($middlex <= 1 or ...) {
                return;
        }
        
        ...;
        
}

##</code><code>##

c:\@Work\Perl\monks>perl -MData::Dump -le
"use warnings;
 use strict;
 ;;
 my $br = build_rec(100);
 dd 'build_rec returned', $br;
 ;;
 sub build_rec {
   my ($middlex) = @_;
   ;;
   my $middlex /= 2;
   if ($middlex <= 1) { return; }
   ;;
   print qq{did not return. will do something useful.};
   return $middlex * 1_000;
   }
"
"my" variable $middlex masks earlier declaration in same scope at -e line 1.
Use of uninitialized value in division (/) at -e line 1.
("build_rec returned", undef)