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 $_.

The lexical::underscore function returns a scalar reference to either a lexical
$_ variable somewhere up the call stack (using PadWalker magic), or to the
global $_ if there was no lexical version.

Wrapping lexical::underscore in ${ ... } dereferences the scalar reference,
allowing you to access (and even assign to) it.