Starting with Perl 5.10, it is possible to create a lexical version of
the Perl default variable $_. Certain Perl constructs like the "given"
keyword automatically use a lexical $_ rather than the global $_.

It is occasionallly useful for a sub to be able to access its caller's
$_ variable regardless of whether it was lexical or not. The "(_)" sub
prototype is the official way to do so, however there are sometimes
disadvantages to this; in particular it can only appear as the final
required argument in a prototype, and there is no way of the sub
differentiating between an explicitly passed argument and $_.

This caused me problems with Scalar::Does, because I wanted to enable
the "does" function to be called as either:

does($thing, $role);
does($role); # assumes $thing = $_

With "_" in the prototype, $_ was passed to the function at the end of
its argument list; effectively "does($role, $thing)", making it
impossible to tell which argument was the role.

Enter "lexical::underscore" which allows you to access your caller's
lexical $_ variable as easily as:

${lexical::underscore()}

You can access lexical $_ further up the call stack using:

${lexical::underscore($level)}

If you happen to ask for $_ at a level where no lexical $_ is available,
you get the global $_ instead.

This module does work on Perl 5.8 but as there is no lexical $_, always
returns the global $_.