I think smls might be on the right track with moving pos. This benchmark seems to show that there is, if anything, a performance gain. I tried a modification to my first attempt my removing the duplicate matches via a hash but it only showed a marginal improvement in performance.

Then again, I am not very good at benchmarks so I could have cocked it up :-/

use strict;
use warnings;

use 5.014;

use Benchmark qw{ cmpthese };

my $n = 4;

my $str = q{x} x 50;
substr $str, $_, 4, q{fred} for 4, 9, 20, 24, 31, 40;
say qq{String: $str\n};

my $rcMovePos = sub {
   my $raMatches;
   while ( $str =~ m{\G(?:.{$n})*?(?=(fred.*))}g )
   {
       push @{ $raMatches }, [ pos( $str ), $1 ];
       pos $str += $n;
   }
   return $raMatches;
   };

my $rcNoDups = sub {
   my $rhMatches;
   $rhMatches->{ pos( $str ) } = $1 while
      $str =~ m{\G(?:.{$n})*?(?=(fred.*))}g;
   return $rhMatches;
   };

my $rcWithDups = sub {
   my $raMatches;
   push @{ $raMatches }, [ pos( $str ), $1 ] while
      $str =~ m{\G(?:.{$n})*?(?=(fred.*))}g;
   return $raMatches;
   };

my $raRes = $rcMovePos->();
say  q{Using $rcMovePos};
say qq{  Matched $_->[ 1 ] at position $_->[ 0 ]} for @{ $raRes };

my $rhRes = $rcNoDups->();
say  q{Using $rcNoDups};
say qq{  Matched $rhRes->{ $_ } at position $_} for
   sort { $a <=> $b } keys %{ $rhRes };

$raRes = $rcWithDups->();
say  q{Using $rcWithDups};
say qq{  Matched $_->[ 1 ] at position $_->[ 0 ]} for @{ $raRes };

srand 1234567890;

$str = q{x} x 10000;
substr $str, int rand 9997, 4, q{fred} for 1 .. 50;

say q{};
cmpthese(
   -5,
   {
       movePos  => $rcMovePos,
       noDups   => $rcNoDups,
       withDups => $rcWithDups,
   } );
[download]

String: xxxxfredxfredxxxxxxxfredfredxxxfredxxxxxfredxxxxxx

Using $rcMovePos
  Matched fredxfredxxxxxxxfredfredxxxfredxxxxxfredxxxxxx at position 4
  Matched fredfredxxxfredxxxxxfredxxxxxx at position 20
  Matched fredxxxfredxxxxxfredxxxxxx at position 24
  Matched fredxxxxxx at position 40
Using $rcNoDups
  Matched fredxfredxxxxxxxfredfredxxxfredxxxxxfredxxxxxx at position 4
  Matched fredfredxxxfredxxxxxfredxxxxxx at position 20
  Matched fredxxxfredxxxxxfredxxxxxx at position 24
  Matched fredxxxxxx at position 40
Using $rcWithDups
  Matched fredxfredxxxxxxxfredfredxxxfredxxxxxfredxxxxxx at position 4
  Matched fredxfredxxxxxxxfredfredxxxfredxxxxxfredxxxxxx at position 4
  Matched fredfredxxxfredxxxxxfredxxxxxx at position 20
  Matched fredfredxxxfredxxxxxfredxxxxxx at position 20
  Matched fredxxxfredxxxxxfredxxxxxx at position 24
  Matched fredxxxfredxxxxxfredxxxxxx at position 24
  Matched fredxxxxxx at position 40
  Matched fredxxxxxx at position 40

           Rate withDups   noDups  movePos
withDups 2321/s       --      -3%     -33%
noDups   2394/s       3%       --     -31%
movePos  3445/s      48%      44%       --
[download]

I hope this is of interest.