undef is false by definition in Perl and so is absolutely fine whenever in a boolean context.

"By definition" doesn't help explain it. For example, undef is interpreted as 0 by definition in a numeric context, but I nevertheless get "Use of uninitialized value" for 1 + undef.

perldiag, explaining the message, just says 'An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake.' Since there is no qualifier on "used", I interpreted it to include use in a boolean context. I'd love to know where it might be clarified in documentation.

Hmm, now looking at it again, maybe the second sentence is the real rule (so the first is a misleading summary): you get the warning if and only if, due to the operation it's being used in, Perl has to convert an undef value to a string ("") or number (0)? If so, I still think that undef->{anything} not causing an error is a bad design, and other uses in an rvalue context might be. Personally, I want it to complain about any use of an undefined value.

What contexts are there for use of undef? Converting to a string, converting to a string, assignment, boolean, dereferencing via ->, ... any others?

Sub call?

$ perl -e 'my $x = undef; &$x("frog")'
Undefined subroutine &main:: called at -e line 1.

[download]

I guess that's a conversion to a string instead of a bad reference -- like an eval rather than like undef->{...}.

In reply to Re^2: When DOESN'T "Use of uninitialized value" show up? by Tim McDaniel
in thread When DOESN'T "Use of uninitialized value" show up? by Tim McDaniel

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.