Further to what others have said you might want to check that you're able to open the directories e.g

opendir(DIR, $PWD) or return warn("$0: $! [$PWD]");
...
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So you'll get a warning and the sub will return. You may also want to check rename for success.

Of course, you could always do this with File::Find::Rule, the spiritual successor to File::Find, like so

use File::Find::Rule;

rename $_ => "${_}z" or warn "$0: $! [$_]"
  for find(file => in => $ARGV[0]);
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See. the File::Find::Rule docs for more info.
HTH

_________
broquaint