There's nothing built-in, but it's fairly easy to achieve the effect with a regexp.

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;
}
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