しっかりと書くなら
my $referer = exists $ENV{'HTTP_REFERER'} ? $ENV{'HTTP_REFERER'} : '-';
Referer: 0 は明らかに無効だから単に
my $referer = $ENV{'HTTP_REFERER'} || '-';

あと、uri_escape()でやるならURIつかったら
sub {
my $uri = URI->new(GA_PIXEL);
$uri->query_form(
'utmac' => GA_ACCOUNT,
'utmr' => $ENV{'HTTP_REFERER'} || '-',
...
);
return $uri;
}