When you use utf8;, then your string literals are text strings. But this

my $referer_url = "@{[ $q->url ]}";
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tries to interpolate an URL, and that seems to be a byte string. Simply mixing those two is a very bad idea.

I don't see why you do it in this complicated way anyway. What's wrong with

my $url = $q->url;
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or maybe

my $url = decode_utf8 $q->url;
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depending on how you want to use it.