The behaviour you are seeing is 100% expected and correct. getlogin() is meant to return null if the process is run with no controlling terminal, and that's what you're doing ;)

The getlogin() syscall returns the user associated with the controlling terminal for the process, or null if there is no controlling terminal.

The way you are running perl, via sh -c, with an & to background it, results in the process having no controlling terminal.

You can more simply see this behaviour with the "tty" command, which shows the controlling terminal.

Normally tty returns something like:

/dev/pts/0

But if you run

sh -c "tty &"

You'll see a different output:

not a tty

If you really want to do what you're doing, consider replacing "getlogin()" with "getpwuid($<)" which uses the real user ID rather than the controlling terminal owner, and so works even backgrounded.