% perl -wle 'my %foo; if ( %foo ) { print "OK" } else { print "KO" }'
KO
% perl -wle 'my %foo = (1,2); if ( %foo ) { print "OK" } else { print "KO" }'
OK

##</code><code>##

% perl -wl
print( %foo ? "OK" : "KO" );
my @bar = grep $_, @foo{ 1, 2, 3 };
print( %foo ? "OK" : "KO" );
^D
KO
OK

##</code><code>##

if ( grep defined, values %foo ) {
  # etc.
}