I wanted to write a sub that defaulted to operating upon $_ per chop, chomp and others.

The problem I had was that I tried:

sub x(_) {
    my $s = shift;
    ## make changes to $s ...
}
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but in the calling code, $_ remained unmodified.

As soon as I saw Corion's example above, it became obvious that when assigned to a named var, the named var held a copy rather than an alias; and that to achieve my goal, I would need to use $_[n]; just as with a normal subroutine: but not before seeing it. Silly perhaps, but ...

I think the addition of a hint about aliasing or a short, working example would strengthen the documentation and perhaps save other people the same frustration.