Set an environment variable, "PERL_MTYEST=''", and then run the following script:

use feature qw/say/;


say exists($ENV{PERL_MYTEST})  ? "exists"          : "doesn't exist";
say defined($ENV{PERL_MYTEST}) ? "defined"         : "undefined";
say length($ENV{PERL_MYTEST})  ? "non-zero length" : "zero length";
say "<<$ENV{PERL_MYTEST}>>";

[download]

The output will be:

exists
defined
zero length
<<>>

[download]

Which indicates that setting an environment variable to '' (an empty string) does not create an undefined variable, but rather, a defined variable with a value of zero length. Your assumption is just incorrect, that's all. Test for length.

If you unset the environment variable in question, and then run the same test, you will get "doesn't exist", and "undefined".

I suppose it's possible that your mileage will vary depending on operating system, but I don't know what OS would do it differently.

Dave

In reply to Re^4: not able to make defined-or operator work in my code by davido
in thread RE: not able to make defined-or operator work in my code by lonewolf28

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.