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copied!<p>Fundamentally, a hash is a data structure optimized for solving the converse question, knowing whether the <em>key</em> 2 is present. But it's hard to judge without knowing, so let's assume that won't change.</p> <p>Possibilities presented here will depend on:</p> <ul> <li>how often you need to do it</li> <li>how dynamic the hash is</li> </ul> <h2>One-time op</h2> <ul> <li><code>grep $_==2, values %x</code> (also spelled <code>grep {$_==1} values %x</code>) will return a list of as many 2s as are present in the hash, or, in scalar context, the number of matches. Evaluated as a boolean in a condition, it yields just what you want.<br> <a href="http://perldoc.perl.org/functions/grep.html" rel="noreferrer"><code>grep</code></a> works on versions of Perl as old as I can remember.</li> <li><code>use List::Util qw(first); first {$_==2} values %x</code> returns only the first match, <code>undef</code> if none. That makes it faster, as it will short-circuit (stop examining elements) as soon as it succeeds. This isn't a problem for 2, but take care that the returned element doesn't necessarily evaluate to boolean true. Use <code>defined</code> in those cases.<br> <a href="http://perldoc.perl.org/List/Util.html#first-BLOCK-LIST" rel="noreferrer"><code>List::Util</code></a> is a part of the Perl core since 5.8.</li> <li><code>use List::MoreUtils qw(any); any {$_==2} values %x</code> returns exactly the information you requested as a boolean, and exhibits the short-circuiting behavior.<br> <a href="http://search.cpan.org/perldoc/List::MoreUtils" rel="noreferrer"><code>List::MoreUtils</code></a> is available from CPAN.</li> <li><code>2 ~~ [values %x]</code> returns exactly the information you requested as a boolean, and exhibits the short-circuiting behavior.<br> <a href="http://perldoc.perl.org/perlsyn.html#Smart-matching-in-detail" rel="noreferrer">Smart matching</a> is available in Perl since 5.10.</li> </ul> <h2>Repeated op, static hash</h2> <p>Construct a hash that maps values to keys, and use that one as a natural hash to test key existence.</p> <pre><code>my %r = reverse %x; if ( exists $r{2} ) { ... } </code></pre> <h2>Repeated op, dynamic hash</h2> <p>Use a reverse lookup as above. You'll need to keep it up to date, which is left as an exercise to the reader/editor. (hint: value collisions are tricky)</p>

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