I beleive you can just declare $fix in the scope of the FOO package instead of the sub. The following appears to do what you want.

use strict;
use warnings;


{
  package FOO;
  my $fic;
  
  sub Principal::bof {
      $fic = "toto";
      bof2();
  }
  
  sub bof2 {
      print $fic;
  }
}

Principal::bof();
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