1. 半径为12.5时，圆的周长应该是多少？
#!/usr/bin/perl
$radius = 12.5;
print $radius * 2 * 3.141592654."\n";
2. 修改上题，提示用户键入半径，求出圆的周长。
#!/usr/bin/perl
print "Please input the radius of the circle: ";
$radius = <STDIN>;
print "The perimeter of the circle is: ";
print $radius * 2 * 3.141592654."\n";
3. 修改上题，当用户输入小于0的半径时，输出0而不是负数。
#!/usr/bin/perl
print "Please input the radius of the circle: ";
$radius = <STDIN>;
print "The perimeter of the circle is: ";
if ($radius lt 0) {
print "0"."\n";
}else {
print $radius * 2 * 3.141592654."\n";
}
4. 提示用户输入两个数字，求两者的乘积？
#!/usr/bin/perl
print "Please input the first number: ";
$num_1 = <STDIN>;
print "Please input the second number: ";
$num_2 = <STDIN>;
$result = $num_1 * $num_2;
print "The product of the two numbers is: $result\n";
5. 用户键入一个字符串和一个数字，以数字为重复次数，连续输出字符串。
#!/usr/bin/perl -W
print "Please input a string: ";
$string = <STDIN>;
print "Please input a positive number: ";
$num = <STDIN>;
$result = $string x $num;
print "The result is:\n$result";
