sub mycode {
    (my $var = $_[0]);
    (defined($var) ? print("mycode() called with $var\n") :
print("mycode() called with no operands.\n"));
    ($var and (my $foo = 'blah'));
#^^^^^^^^^^^^^^^ Look here
    ($foo and print("foo is $foo\n"));
    ($foo ||= 'FOO');
}
mycode('test');
mycode();
mycode();
mycode();
a.pl syntax OK