As far as I'm aware, there isn't currently a supported introspection tool for determining a lexical var's type.

As far as I'm aware, fields isn't deprecated.

As far as I'm aware, the only compile-time checking that the perl core does for lexical is types is that the package corresponding to the type must already exist, and that in conjunction with the compile-time use of fields, it checks constant hash lookups of a typed lexical reference. For example:

package Foo;

use fields qw(a b c);
sub new { fields::new(@_); }

package main;

my Foo $r = Foo->new();
$r->{d} = 1;    # compile-time error
$key = 'd';
$r->{$key} = 1; # run-time error
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Dave.