I wish to discuss your second example first. undef is a special 'value'. It is the default value given to a scalar when it is created. It can be assigned later, as you have shown. Its behavior depends on context.

As a number, it is treated as zero. AS a string, it is treated as a null string. As a Boolean, it is treated as 'false'. As a argument to defined, it is the only value for which defined will return 'false'. 

?perl -e"$line = undef; print 3 + $line"
3
?perl -e"$line = undef; print '3' . $line"
3
?perl -e"$line = undef; print 3 unless $line"
3
?perl -e"$line = undef; print 3 unless defined $line"
3

[download]

Your first example does not do what you think. try it. You probably mean: 

$current_time='14:30:05'
(undef, $min, $sec)=split /\:/, $current_time

[download]

I think of this as an idiomatic use which means exactly what you describe. That field of the assignment is ignored.

Bill

In reply to Re: Two meanings of undef by BillKSmith
in thread Two meanings of undef by likbez

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.