This was recently actually documented in perlop by David Nicol: 
=head2 No-ops

Perl doesn't officially have a no-op operator, but the bare constants
C<0> and C<1> are special-cased to not produce a warning in a void
context, so you can for example safely do

    1 while foo();

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In reply to Re: Sometimes it's in Void Context by ysth
in thread Sometimes it's in Void Context by Ovid

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