Perl Learning Experience (note)

To include double quotes or backslashes in the string, add a backslash before it, and the backslash can also cancel the variable substitution

Single quotes another feature that can span multiple lines, such as:
$text = ' This is two
Lines of text
';
is equivalent to the following sentence:
$text = "This is two/nlines of text/n";

In Perl, all simple variables have the default initial value: "", which is the null character.

Conversion of strings and values
Example 1:
$string = "43";
$number = 28;
$result = $string + $number; # $result = 71
If the string contains non-numeric characters, it starts from left to the first non-numeric character, such as:
$result = "Hello" * 5; # $result = 0
$result = "12a34" +1; # $result = 13

Arithmetic operators: + (plus),-(minus), * (multiply),/(except), * * (Power),% (take more),-(Monocular negative)
(1) The cardinality of the power can not be negative, such as ( -5) * * 2.5 # error;
(2) The result of power can not exceed the limit of computer expression, such as 999999 # error
(3) The operand of the remainder, if not an integer, rounded to an integer; the right-hand side of the operator cannot be zero
(4) Monocular negative can be used for variables:-$y; # equivalent to $y *-1

The operator <=> result is:
0-Two values equal
1-The first value is large
-1-The second value is large

string comparison operator

string comparison operator

Operator	Describe
Lt	Less than
Gt	Greater than
eq	Equals
Le	Less than or equal to
Ge	Greater than or equal to
Ne	Not equal to
Cmp	Compare, return 1, 0, or-1

Logical XOR OR: $a xor $b
Bitwise XOR OR: ^
Bit and:& Note: Do not use & for negative integers, because Perl will convert them to unsigned numbers. #。。

In Perl, + + can be used for strings, but when the end character is ' z ', ' z ', ' 9 '

Do not use--,perl will first convert the string to a number and then subtract from it
$stringvar = "ABC";
$stringvar--; # $stringvar =-1 now

If the string contains a non-alphanumeric character, or the number is in the letter, the value of the + + operation is converted to a numeric zero, so the result is 1