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unless ($query->param('is_a_chicken')) # Fails on 0
{
     print "You must choose an option.<BR>\n";
}

unless (length $query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}

my $is_a_chicken = $query->param('is_a_chicken');
unless (defined ($is_a_chicken) && length ($is_a_chicken))
{
     print "You must choose an option.<BR>\n";
}

sub provided
{
     return defined($_[0]) && length ($_[0]);
}

unless (provided ($query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}