perl's Favorite Default: $_

If you omit the control varibele from the beiginning of foreach loop, Perl uses its favrorite default variable $_. This is (mostly) just like any other scalar variable, except for its unusual name. For example:

..) { #use $_ by default
  print "I can count to $_!\n";
}

Althongh this isn't Perl's only default by a long shot, it's Perl's most common default. You will see many other cases in which Perl will automatically use $_ when you don't tell it to use some other variable or value, thereby saving the programmer from the heavy labor of having to thinking up and type a new variable name. So as not to keep you in suspense, one of those case is print, which will print $_ if given no other argument:

$_ = "yaboo dabba doo";
print;  #print $_ by default

From p54 of the book named Learning Perl

loop 圈，环，回来。在程序语言里，应该译为“循环”，比如for循环。

scalar 数量，标量。在perl里，就是指的这种数据类型，singular，区别于列表（list）。

not by a long shot 绝不、远没有

in suspense  悬而未决，处于焦灼等待状态，心神不宁;吊胃口