Not quite - not documenting them is just one way to hide variables.
It is the official Perl way, as envisioned by Larry Wall. (I should have said "members" rather than "variables". Hiding variables is done by scoping.) Any other technique used to hide private methods and members constitutes an attempt to write C++ (or some other language) in Perl.
it makes placing an underscore before a private variable a secondary security measure
It's really just a flag to help the module author (and any developer with prying eyes) keep things straight. Nothing more. I agree that it's a good convention.
Caution: Contents may have been coded under pressure.

In reply to Re^6: I hate the leading underscores. by Roy Johnson
in thread I hate the leading underscores. by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.