I think I must be missing something about the if bit - it doesn't seem to do anything. If that's right, then you could get the same result with 
print "$in was not a number!"; input() unless $in =~ /\d/;

[download]
which does get it on one line, though not, I agree, with a trailing if :). On the other hand if the line 
            $in;

[download]
is a line you need (perhaps an idiom I don't know - if so, what does it do? - in any event it has a certain classically simplicity about it) - then to get it on one line you could use the conditional operator, although there might be some pros and cons here I don't know about: 
$in =~ /\d/ ? $in : print "$in was not a number!";

[download]
That doesn't let you call input(), but that's ok if you agree with lesstrat that it might get a bit ropey if you call input() from within input().

On the broader question of how to detect numbers, I find that what I usually want to isolate is everything that isn't not a number, so my favourite idiom is 
$in !~ /\D/

[download]
But this question is much more complicated than one would think, as these and threads explain.

§ George Sherston

In reply to Re: Can this If/Else be condensed into a 1 liner with a trailing If? by George_Sherston
in thread Can this If/Else be condensed into a 1 liner with a trailing If? by jerrygarciuh

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.