I don't see why you're asking this since you've since specified you're talking about undef the function and not undef the value, but it's easy to pass undef by accident: 

substr($s,1,2,`foo`)

[download]
$s = substr($s,0,1) . `foo` . substr($s,3);

[download]

That's all I said concerning passing undef by accident.

I don't know why you're asking me to explain something you said.

I don't know why you're implying I said one might write "undef" by accident.

Update: Completely failed at answering parent due to inability to make sense of it, so changed to say just that.

In reply to Re^7: Use of uninitialized value in substr by ikegami
in thread Use of uninitialized value in substr by BrowserUk

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.