I have this `trim` routine:

```
sub trim {
    for (@_) {
        s|^\s+||;
        s|\s+$||;
    }
}

```

It trims whitespace "in-place", i.e.:

```
trim $x, $y;

```

will trim the whitespace of $x and $y (modifying them).

How can I improve `trim` so that it lets me call it like:

```
trim;

```

and that will be the same as:

```
trim $_;

```

?