Because doing it by hand, even by an expert, is so error prone (not to mention completely illegible):

use strict;
use warnings;
use 5.010;

use Number::Format;

my $number = 1222333444.01234567;

my $formatter = new Number::Format(
    -decimal_digits => '4',
    -decimal_point => '!',
);

say $formatter->format_number($number);

--output:--
1,222,333,444!0123
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Or, copy the solution from perlfaq.