It'd be helpful to know more of the specifics. For example, if the keys are limited to alphanumeric labels, the simplification could provide further speedup. Anyway, here's one regex-convoluted aka "adding problems to problems" version for your perusal:

sub uhuu {
    my $keys = join "|", map "\Q$_\E", ($match_list =~ m{([^/,]+)/[^,]
+*}g);

    my %R;
    return unless $attrs =~ m{
        \A \Q$tag\E \s+
        (?: ($keys)=(\S+) (?(?{ defined $R{$1} || ($R{$1}=$2, 0) })(?!
+)) \s* )*+
        \z }x;

    $markup =~ s[X{($keys)}][ $R{$1} // $& ]ger
}
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A side-note to benchmarkers: localize the globals that you modify. ("On the importance of testing your testing.")