use warnings;
use strict;

my @numb3rs = qw/1024 16777215 3,1415926 42 759092417205229390873,8763617/;

for my $num ( @numb3rs ) {
    print "number: $num\n";
    my $regexed = $num;
    $regexed =~ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1./g;
    print "perlfaq: $regexed\n";
    $regexed = $num;
    $regexed =~ s/(\d+)(\d{3})/$1.$2/;
    print "strat:   $regexed\n";
    $regexed = $num;
    $regexed =~ s/(\d\d\d)(?=\d)(?!\d*,)/$1./g;
    print "crian:   $regexed\n";
} continue {
    print '=' x 75, "\n"
}

peters@ronin:~> perl test.pl
Possible attempt to separate words with commas at test.pl line 4.
number: 1024
perlfaq: 1.024
strat:   1.024
crian:   102.4
===========================================================================
number: 16777215
perlfaq: 16.777.215
strat:   16777.215
crian:   167.772.15
===========================================================================
number: 3,1415926
perlfaq: 3,1415926
strat:   3,1415.926
crian:   3,141.592.6
===========================================================================
number: 42
perlfaq: 42
strat:   42
crian:   42
===========================================================================
number: 759092417205229390873,8763617
perlfaq: 759.092.417.205.229.390.873,8763617
strat:   759092417205229390.873,8763617
crian:   759092417205229390873,876.361.7
===========================================================================