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

$ perl commify.pl
               -0.0001234567890 commified becomes -0.000123456789
               -0.0012345678900 commified becomes -0.00123456789
...
  -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