sub commify {
   local $_  = shift;
   s{(?<!\d|\.)(\d{4,})}
    {my $n = $1;
     $n=~s/(?<=.)(?=(?:.{3})+$)/,/g;
     $n;
    }eg;
   return $_;
}

##</code><code>##

sub commify {
  local $_ = shift;
  s{(?:(?<=^)|(?<=^-))(\d{4,})}
   {my $n = $1;
    $n=~s/(?<=.)(?=(?:.{3})+$)/,/g;
    $n;
   }e;
  return $_;
}

##</code><code>##

sub commify {
  local $_ = shift;
  if (/^-?(\d{4,})/g) {
    for (my $p=$+[1]; $p>$-[1]; $p-=3) {
      substr($_,$p,0) = ','
    }
  }
  return $_;
}