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#      0    5   10   15   20   25   30   35
#      '    '    '    '    '    '    '    '
$_ = q{.....fred1..fred2...fred3....fred4..};
...
    say "  matched 'fred$1' at pos @{[pos($_)-$n]} (gobbled '$&')";
  }
}

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