#      0    5   10   15   20   25   30   35
#      '    '    '    '    '    '    '    '
$_ = q{.....fred1..fred2...fred3....fred4..};
#      ----++++----||||----||||----++++----  $n = 4
#      -----||||+-----+++++||||-+++++-----+  $n = 5

my $capture = qr([0-9]\.+);  # the (....) in the OP's specification

for my $n ( 4, 5 )
{
  say "\$n = $n";
  while ( m[\G(?:.{$n})*?(?=fred($capture)).{0,$n}]g ) {
    say "  matched 'fred$1' at pos @{[pos($_)-$n]} (gobbled '$&')";
  }
}

##</code><code>##

$n = 4
  matched 'fred2...' at pos 12 (gobbled '.....fred1..fred')
  matched 'fred3....' at pos 20 (gobbled '2...fred')
$n = 5
  matched 'fred1..' at pos 5 (gobbled '.....fred1')
  matched 'fred3....' at pos 20 (gobbled '..fred2...fred3')