my $sep    = ',';
my $number = '12345678.27';

print MakeThousands($number), "\n";

sub MakeThousands {
  local $_ = reverse(shift());
  s/(\d{3})(?=\d)(?!\d*\.)/$1$sep/go;
  return scalar(reverse());
}

##</code><code>##

Benchmark: timing 5000000 iterations of MakeThousands, Thousandformat...
MakeThousands: -1 wallclock secs ( 0.06 usr +  0.00 sys =  0.06 CPU) @ 83333333.33/s (n=5000000)
Thousandformat:  1 wallclock secs ( 0.12 usr +  0.00 sys =  0.12 CPU) @ 41666666.67/s (n=5000000)