A search for "commify" yields plenty of reading material.

update

Here's the code I use which originated (IIRC) from comp.lang.perl.misc many years ago:

sub commify {
    return $_[0] if $_[0] !~ m/^\d*?[.]?\d*$/;
    ( my $input = reverse 0+$_[0] ) =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>
+g;
    return scalar reverse $input;
}
[download]

Not built-in....

my $text = 12345678.9;

commify($text);

print "\n |$text|\n";

# after Perl Cookbook recipe 2.17
sub commify {
     my $text = reverse $text;
     $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
     print "In the sub: " . reverse $text . "\n";
}
[download]

FWIW, searching for "commafy" will also lead you to a thread with a useful node ...and, there's help at the ends of your fingers as well:
see perldoc -q comma.