I understand that doing 
while (<FH>) ...

[download]
is the same as doing 
while ($_=<FH>) ...

[download]
which, in turn, is the same as doing 
while (defined($_=<FH>)) ...

[download]
so far, so good. But then why the camel book says that this is suboptimal: 
if ($_=<FH>) ...

[download]
and this is optimal: 
if (defined($_=<FH>)) ...

[download]
As far as I understand, $_=<FH> will assign to $_ the line read from FH, or an undefined value if we happen to be at the end of file FH. undefined is evaluated as false in boolean context. So why is the first idiom "suboptimal"? Thanks for any help.

Update: Ok, many thanks to everybody for your replies.

In reply to Doubt on defined-ness by waldner

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.