How is the command executed if the shell is not invoked?

By asking to system execute the specified program instead of asking the system to execute /bin/sh.

Why does adding a redirect to /dev/null increase the complexity enough to invoke the shell?

There's no point in including an entire shell in Perl. I don't know exactly what part of shell command parsing is implemented in Perl.

Finally, what method is used to execute the command if I had instead used perl's system() function instead of the backticks?

system($cmd), exec($cmd), open(my $pipe, '-|', $cmd) (and its 2-arg form) and open(my $pipe, '|-', $cmd) (and its 2-arg form) work the same way as `$cmd` (aka qx`$cmd` aka readpipe($cmd)).

system($prog, LIST) where LIST returns at least one scalar won't invoke the shell.^[1]

system({ $prog } $prog, LIST) won't invoke the shell.

1. It can in Windows.