Why the need for sprintf ?? Am I missing something of importance?

If sprintf is not needed, here is one at 49 chars....

sub a{$_=pop;{s/(\d)(\d{3})(?!\d)/$1,$2/&&redo}$_.'.00'}
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update: I am a little confused about the input, the post asks for an integer, which would not have any decimal places, but some of the input has a decimal place..
Should this handle input with decimal places, or should it just trust that the user submits integers only?

perl -e '$_=$0;split??;chop$_;$;=pop@_;$;++for(0..9060420);$_.=reverse
+$;;print'
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