Under normal usage, they take values not variables. If you run the code

sub add_one {
    my $x = shift;
    $x += 1;
    return $x;
}
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You would not expect the variable you passed in (my $y = add_one($outer_x);) to change value, right? This is a little awkward/contrived, but this is in contrast to more object-oriented frameworks.

Advanced: Of course Perl actually does pass by reference with lvalues bound to @_. But that's probably the best reason for transferring subroutine inputs to locally-scoped variables.