I agree that this is a case for sexeger.

I ran your 'commify' sub through BrowserUk's commify tester above, and noticed that it could use a zero-width positive lookahead assertion (?=\d+) instead of the direct match of a digit after the (\d{3})(\d) (that almost works, but leaves some commification with 4 digits between comma's). And, since it uses a lookahead, the final $2 can be removed from the substitution. Here's my version of commify sexeger...

sub commify
{
    local $_ = reverse shift;
    /\./g;
    s/\G(\d{3})(?=\d+)/$1,/g;
    return scalar reverse $_;
}
[download]

Here's the results with the updated sexeger:

$ perl commify.pl
               -0.0001234567890 commified becomes -0.000123456789
               -0.0012345678900 commified becomes -0.00123456789
               -0.0123456789000 commified becomes -0.0123456789
               -0.1234567890000 commified becomes -0.123456789
               -1.2345678900000 commified becomes -1.23456789
              -12.3456789000000 commified becomes -12.3456789
             -123.4567890000000 commified becomes -123.456789
            -1234.5678900000000 commified becomes -1,234.56789
           -12345.6789000000008 commified becomes -12,345.6789
          -123456.7890000000043 commified becomes -123,456.789
         -1234567.8899999998976 commified becomes -1,234,567.89
        -12345678.9000000003725 commified becomes -12,345,678.9
       -123456789.0000000000000 commified becomes -123,456,789
      -1234567890.0000000000000 commified becomes -1,234,567,890
     -12345678900.0000000000000 commified becomes -12,345,678,900
    -123456789000.0000000000000 commified becomes -123,456,789,000
   -1234567890000.0000000000000 commified becomes -1,234,567,890,000
  -12345678900000.0000000000000 commified becomes -12,345,678,900,000
 -123456789000000.0000000000000 commified becomes -123,456,789,000,000
-1234567890000000.0000000000000 commified becomes -1.23456789e+15
[download]

Cheers!