This will give the answer without the superfluous "B::" prefix:

use feature qw( say );
use B qw( );

my ($f,$g) = (333,"xyz");
say B::class(B::svref_2object(\$f));
say B::class(B::svref_2object(\$g));
[download]

IV
PV
[download]

Note that this returns the SV *type*. It can return "PV" even if the scalar has no PV.

use feature qw( say );
use B qw( );

$_ = 333;
"".$_;
$_ = undef;
say B::class(B::svref_2object(\$_));
[download]

PVIV
[download]

This is what the OP asked, but he probably wants to know what the scalar *has*.