my @ss = ("a","b","c","d");
print $ss[1]  显示为a
print @ss[1,3] 显示为a c
@ss的标量为4
if (@ss < 5)
{
command
}
else
{
command
}

if($date == $today)
{
command;
}
elseif($date == $christmas)
{
command;
}
else
{
command;
}

print "happy unbirthday" unless ($date == $today);

unless ($date != $today)
{
command;
}
else
{
command;
}
IF……ELSE

($date == $today) ? print "Happy B.Day!\n" : print "Happy Day!\n";