The following are all be equivalent and valid:

• \xEF\xBB\xBF<?xml version="1.0" encoding="UTF-8"?>
• <?xml version="1.0" encoding="utf-8"?>
• <?xml version='1.0' encoding='UTF-8'?>
• <?xml version="1.0"?>
• <?xml encoding="UTF-8" version="1.0"?>
• <?xml version="&#x31;&#x2E;&#x30;" encoding="&#x55;&#x54;&#x46;&#x2D;&#x38;"?>

And that's assuming you're just supporting UTF-8!

This isn't the job of a regex. But if you insist on using a regex match, here you go:

use XML::LibXML qw( );

local our $doc;
$content =~ /(?(?{ !eval {
    $doc = XML::LibXML->new()->parse_string($_)
} })(?!))/;
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The match will succeed if the parsing succeeds (and set $doc to a XML::LibXML::Document object) and fail if the parsing fails.