As gaal said 42 is a noop, just like any other constant in void context. I mean if you have a program where a constant is used as an statement like: 

"me";
9999;
{ throw => 'away' };

[download]
this program surely does nothing. By the way, if you ask for warnings, Perl will tell you so: 
$ perl -w -e 42
Useless use of a constant in void context at -e line 1.

[download]

The one-liner is a noop over the standard input, which in this case comes from the pipe echo test |. So it allows you to do nothing, while consuming the input.

As a side note, there's nothing special about 42 except it is the Answer to The Ultimate Question Of Life, the Universe and Everything in pop culture. Very common in humorous pieces of code.

In reply to Re^3: expression which does nothing by ferreira
in thread expression which does nothing by jesuashok

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.