# Non-obfuscated form, using tie():
use Interpolation;
my %commify;
tie %commify, Interpolation => sub {
    local $_ = scalar reverse shift;
    s/(\d+\.)|(\d\d\d)(?=\d)/$1 || "$2,"/ge;
    return scalar reverse $_;
};

$a = 12345.21;
$b = 1357.98;
print "$a*$b with commas inserted looks like '$commify{$a*$b}'.\n";