The following code will comma-fy floating numbers, like 10000.00 into 10,000.00. Which I think would be more useful to you than one that formats just integers.

{
  local $_ = $amount;
  /\./ ? s/(?<=\d)(?=(\d{3})+(?:\.))/,/g :
         s/(?<=\d)(?=(\d{3})+(?!\d))/,/g;
  $amount = $_;
}
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