It's working alright, you're changing the value of @ARGV, but you're not doing anything with the value it returns.

You seem to be expecting it'll put its return value into $_ when in void context, but it doesn't. I can't even think of any built-in function that does that, for the moment, though split clobbers up @_ in scalar and void context — and that usage is deprecated. Using $_ as a default argument: yes; as a return value: no.

Try:

$_ = shift @ARGV;
print;             # This does use $_ by default
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