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";
}
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Update: As FunkyMonk noted (types faster than I): </update>
Commify numbers, the boring and straightforward way
and
"Commifying" a number
may shed more light on this matter... as will his suggestion to use "Search" as a first step to obtaining the wisdom here in the Monastery.

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.