After useful tips given to me, I've searched Perl distribution perl@16267 and found out that perl571delta.pod says:

=item *

Devel::Peek now has an interface for the Perl memory statistics
(this works only if you are using perl's malloc, and if you have
compiled with debugging).
[download]

... and Devel::Peek has similar explanations.

So, if one interested in such statistic, (s?)he should recompile perl with debugging and perl-malloc options.

A bit pity that this is not available in standard executables, or at least without -DDEBUGGING, but it is good to have such possibility at all.

Many thanks to all who helped me with this,
Vadim.