hello, this seems exactly what docs says(before dive into the source):

@PDF::Labels:PageFormats is a list of known page formats.  Each
        page format is a : delimited list of fields which provide the
        following properties about a page of labels:
...

 $pdf = new PDF::Labels(
                        $PDF::Labels::PageFormats[0],
                                filename=>'labels.pdf',
                                Author=>'PDF Labelmaker',
                                Title=>'My Labels'
                );
[download]

I read TFM as follow: the first argument in object creation must be a Page Format. for your convenince there is a global variable in the class that collect an array of common formats. Even if this seems weird is a reasonable manner to do configuration.
You can also modify only the font size of one of the format in this, not so elegant, way:

my $cust_page_format = $PDF::Labels::PageFormats[0];
my $needed_fontsize = 8;
$cust_page_format =~ s/:\d+(:\d+)$/:$needed_fontsize\1/;
[download]

L*