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my $sep    = ',';
my $number = '12345678.27';
...
  s/(\d{3})(?=\d)(?!\d*\.)/$1$sep/go;
  return scalar(reverse());
}

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)