[tantalor]

Let's unpack that,

    rand($.) < 1 && ($line = $_) while <>;

We all know rand and while, but if you don't know perl the rest is hard.

<> is a common way to read stdin, and the value is assigned to the $_ special variable.

The one I didn't know was $., the current input line number. In other words, its your loop index that automatically increments.

http://www.kichwa.com/quik_ref/spec_variables.html

[ceautery]

That's good until your input stream exceeds 32768 lines [1]. There are better generators available from CPAN, like Math::TrulyRandom.

[1] - http://www.perl.com/doc/FMTEYEWTK/random