my $_; is intended as a replacement for the not-ideal
local $_; or
local *_;.
perl591delta:
=head2 Lexical $_
The default variable $_ can now be lexicalized, by declaring it like
any other lexical variable, with a simple
my $_;
The operations that default on $_ will use the lexically-scoped
version of $_ when it exists, instead of the global $_.
In a map or a grep block, if $_ was previously my'ed, then the
$_ inside the block is lexical as well (and scoped to the block).
In a scope where $_ has been lexicalized, you can still have access to
the global version of $_ by using $::_, or, more simply, by
overriding the lexical declaration with our $_.
