• illguts
• perlapi
• Devel::Peek
• -MO=Concise (More terse and more useful than -MO=Terse.)

Your use of "is" is incorrect when talking about scalar types. An IV is not necessarily an integer.

$ perl -MB -E'$_=123; $_=undef; say B::class B::svref_2object \$_'
IV
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SV is the base "class" for all Perl variables (not just scalars), but yeah, the "S" stands for "scalar".

Here goes:

use B;
use feature qw( say );
format X =
   Foo
.
say B::class B::svref_2object *X{FORMAT};
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The "P" in "PV" is for "pointer".

Where "and/or" is used, all combinations are possible, with the following exceptions:

• An undefined scalar is one that contains nothing at all, so a scalar cannot contain both undef and something else.
• A scalar cannot contain two or more of the following at a time: a reference, a signed integer and an unsigned integer.
• A scalar cannot contain both of the following at a time: a string of 8 bit chars and a string of 32/64 bit chars

While it is technically possible for some scalars to contain both a reference and something else, Perl doesn't create these, and I don't know how safe it is.