• $_ = reverse(join(',', reverse($_) =~ /.{1,3}/g));
  [download]

  or

  $_ = join(',', split(/(?<=.)(?=(?:.{3})+\z)/, $_));
  [download]

  or

  $_ =~ s/(?<=.)(?=(?:.{3})+\z)/,/g;
  [download]

• perlre and perlretut

Update: Tested. Fixed. I knew I needed that \z.
Update: Added third solution, a simplification of the second.

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
[download]

How to add commas to the following number "1234567" as "1,234,567"? using regex substitution and split (if possible)

perldoc -q comma
[download]

which also points you to: How can I output my numbers with commas added?

Perhaps AM meant for the third link to read: Re: How do I add commas to a number?

As to your second question, this seconds AM's first two reading recommendations. You're looking to learn the difference between "lazy quantifiers" and "possessive quantifiers." If it's easier for you, try reading "Mastering Regular Expressions, Friedl (O'Reilly)" or the "Regular Expressions Pocket Reference, Stubblebine (O'Reilly)"

There are some interesting recipes in this thread.