Also .. you mention " I've tried unlink "$queue/$call_file" and rename "$queue/$call_file", "$queue/pending/$call_file", and neither seems to work as advertised." ...

Just how does it fail? Any error messages? Anything in $! ? Have you tried -f "$queue/$call_file" before you continue.. just to be extra super duper paranoid?

Have any code example where you try to delete something and it fails?

Why do you think it failed? Because afterwards things are not where they're supposed to be? Because.. yeah.. need to see some code.