I don't know what the ksh quoting differences might be, so assuming something rational...

Use @ARGV. If you have quoting problems, use qq// and q// inside the one-liner, and single quotes around the one-liner:

> BLAH=$(perl -e 'for (@ARGV){s/(\d)(?=(\d{3})+(\D|$))/$1\,/g;print qq
+/$_\n/}' 12345678 876543210)
12,345,678
876,543,210

> echo $BLAH
12,345,678 876,543,210
[download]

Note that this form will take multiple values, commafy them, and output them one per line.

If you expect more than integers, see the other answers with decimals and other weirdness on how to change the regex.