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");
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My naming convention:
fn = unqualified file name
qn = qualified file name (i.e. rel or abs path)
fqn = fully qualified file name (i.e. abs path)
And only pass qn/fqn to system functions.

Some of the lines can be combined, but I wanted to be explicit for clarity and demonstration purposes.

Your operating system wants you to be more explicit than your shell wants you to be. You will need to construct the target name then from the directory and the filename:

use File::Spec;

my $target_dir = 'TEST';
my $source = 'foo.txt';

my $target = File::Spec->catfile($target_dir, $source);
rename $source => $target
    or die "Couldn't rename '$source' to '$target': $!";
[download]

Thank you, I think I have down on what I need to do. I was missing those few tidbits.