Don't forget to use the current locale information to determine what the correct separator character is and what positions to group it with.

In fact, perllocale has an example of doing exactly that!example program that rewrites its command-line parameters as integers correctly formatted in the current locale.

        require 5.004;
        use POSIX qw(locale_h);
        # Get some of locale's numeric formatting parameters
        my ($thousands_sep, $grouping) =
             @{localeconv()}{'thousands_sep', 'grouping'};
        # Apply defaults if values are missing
        $thousands_sep = ',' unless $thousands_sep;
        # grouping and mon_grouping are packed lists
        # of small integers (characters) telling the
        # grouping (thousand_seps and mon_thousand_seps
        # being the group dividers) of numbers and
        # monetary quantities.  The integers' meanings:
        # 255 means no more grouping, 0 means repeat
        # the previous grouping, 1-254 means use that
        # as the current grouping.  Grouping goes from
        # right to left (low to high digits).  In the
        # below we cheat slightly by never using anything
        # else than the first grouping (whatever that is).
        if ($grouping) {
            @grouping = unpack("C*", $grouping);
        } else {
            @grouping = (3);
        }
        # Format command line params for current locale
        for (@ARGV) {
            $_ = int;    # Chop non-integer part
            1 while
            s/(\d)(\d{$grouping[0]}($|$thousands_sep))/$1$thousands_se
+p$2/;
            print "$_";
        }
        print "\n";
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