$procpath=fileno($fh);
should be
$procpath='/proc/self/fd/'.fileno($fh);

Also, your directory test is missing quotes.

my $filename;
if (defined(my $fileno = fileno($fh))) {
   if ($^O eq 'linux') {
      my $procpath = "/proc/self/fd/$fileno";
      $filename = readlink($procpath);
      $filename = undef unless -e $filename;
   }
}
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