use File::Spec::Functions qw( catfile );
use File::Basename        qw( dirname );

my $src_qn = $x->{filename};
my $dir_qn = dirname($src_qn);
my $dst_fn = "TEST$ext";
my $dst_qn = catfile($dir_qn, $dst_fn);

rename($src_qn, $dst_qn)
   or die("...: $!\n");