A lookaround method that will work on any numbers embedded in a string (but will not commify after a decimal point):

sub commify {
   local $_  = shift;
   s{(?<!\d|\.)(\d{4,})}
    {my $n = $1;
     $n=~s/(?<=.)(?=(?:.{3})+$)/,/g;
     $n;
    }eg;
   return $_;
}
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If you prefer the FAQ rule of only working on things that begin with digits or -,

sub commify {
  local $_ = shift;
  s{(?:(?<=^)|(?<=^-))(\d{4,})}
   {my $n = $1;
    $n=~s/(?<=.)(?=(?:.{3})+$)/,/g;
    $n;
   }e;
  return $_;
}
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This one does a single pattern match and then uses substr() to insert the commas:

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