use strict;
use warnings;
use 5.010;

#      0    5   10   15   20   25   30   35
#      '    '    '    '    '    '    '    '
$_ = q{....fred1....fred2...fred3....fred4..};

while (/fred(?(?{ pos % 4 != 0 })(?!))(....)/sg) {

}

--output:--
Use of uninitialized value in numeric ne (!=) at (re_eval 1) line 1.
Use of uninitialized value in numeric ne (!=) at (re_eval 1) line 1.
Use of uninitialized value in numeric ne (!=) at (re_eval 1) line 1.