Your second regex works,

I'm not seeing that:

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.

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In reply to Re^3: A regex that only matches at offset that are multiples of a given N? by 7stud
in thread A regex that only matches at offset that are multiples of a given N? by BrowserUk

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