I am afraid I do not have the luxury to discard all non-utf8 input, but I can simplify the code:

if the input is not detected as utf8, just treat it as iso-8859-1

use Text::Unaccent;
use Encode::Detect::Detector;

# my $author = "Sch%F6%E5ttl";
# my $author = "Sch%C3%A9ttl";
# my $author = "Sch%C3%B6ttl";
# my $author = "Sch%F6%F6ttl";
# my $author = "Sch%F6 %F4ttl";
my $author = "teoria elasticit%E0";
$author =~ s/%([a-zA-Z0-9][a-zA-Z0-9])/pack('C',hex($1))/eg;

my $encoding = Encode::Detect::Detector::detect($author);
if($encoding !~ m#utf-8#i){
  $encoding = "iso-8859-1";
}
if($encoding){
  $author = unac_string($encoding, $author);
  print "after unac: $author<br>\n";
}
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Seems like it's working better, any potential problem?