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Perl defines some special variables, usually prefixed with $, @, or %, for example: $_. Many special variables have a long English name; the operating system variable $! can be written as $OS_ERROR. If you want to use the English-named special variables, you need to add **use English;** at the beginning of the program. This allows you to use the descriptive English special variables. The most commonly used special variable is $_, which contains the default input and pattern matching content. The example is as follows: ## Example #!/usr/bin/perl foreach('Google','','Taobao'){print$_; print"n"; } Executing the above program, the output is: GoogleTutorialTaobao In the following example, we do not use $_ to output content: ## Example #!/usr/bin/perl foreach('Google','','Taobao'){print; print"n"; } Executing the above program, the output is: GoogleTutorialTaobao In the example, "Google" is printed first, followed by "", and finally "Taobao". In iteration loops, the string of the current loop is placed in $_, and then output via print. Additionally, when print does not specify an output variable, it defaults to using $_. Here are several places where Perl assumes the use of $_ even if it is not explicitly written: * Various unary functions, including functions like ord() and int(), and all file test operations except "-t" ("-f", "-d"), "-t" defaults to operating on STDIN. * Various list functions, such as print() and unlink(). * Pattern matching operations "m//", "s///", and "tr///" when the "=~" operator is not used. * The default iteration variable for "foreach" loops when no other variable is given. * The implicit iteration variable for grep() and map() functions. * When "while" has only a single condition, and that condition is a test of the result of an operation, $_ is the default location for storing the input record. This does not happen except in the "while" test condition. (Mnemonic: The underscore can be omitted in specific operations.) * * * ## Special Variable Types Based on the usage characteristics of special variables, they can be classified into the following categories: * Global scalar special variables. * Global array special variables. * Global hash special variables. * Global special file handles. * Global special constants. * Regular expression special variables. * File handle special variables. ### Global Scalar Special Variables The following lists all scalar special variables, including both the special character and English form of the variables: $_ Default input and pattern matching content. $ARG $. The current line number of the last file handle read. $NR $/ Input record separator, default is the newline character. If set to undef, it reads until the end of the file. $RS $, Output field separator $OFS $ Output record separator $ORS $" This variable is similar to $,, but applies to interpolating arrays and slice values into double-quoted strings (or similar interpolated strings). Default is a space. $LIST_SEPARATOR $; The separator used when simulating multi-dimensional arrays. Default is "34". $SUBSCRIPT_SEPARATOR $^L The form feed character sent to the output channel. Default is "f". $FORMAT_FORMFEED $: The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format. Default is "n". $FORMAT_LINE_BREAK_CHARACTERS $^A The variable used to store formatted data before printing. $ACCUMULATOR $# The default numeric output format when printing numbers (deprecated). $OFMT $? Returns the status of the last external command. $CHILD_ERROR $! The numeric value of this variable is the errno value, and the string value is the corresponding system error string. $OS_ERROR or $ERRNO $@ The error message from the eval command. If empty, it indicates the last eval command executed successfully. $EVAL_ERROR $$ The process ID of the current Perl script. $PROCESS_ID or $PID $ The effective user ID of the current process. $EFFECTIVE_USER_ID or $EUID $( The real group ID of the current process. $REAL_GROUP_ID or $GID $) The effective group ID of the current process. $EFFECTIVE_GROUP_ID or $EGID $0 Contains the filename of the script being executed. $PROGRAM_NAME $[ The subscript of the first element of an array, default is 0. $] The Perl version number. $PERL_VERSION $^D The value of the debug flag. $DEBUGGING $^E Extended operating system error information in non-UNIX environments. $EXTENDED_OS_ERROR $^F The maximum file descriptor number. $SYSTEM_FD_MAX $^H Syntax checking status activated by the compiler. $^I The value of the built-in control editor. $INPLACE_EDIT $^M The size of the backup memory pool. $^O The operating system name. $OSNAME $^P An internal variable specifying the current debug value. $PERLDB $^T The time in seconds since the script started running, calculated from the new century. $BASETIME $^W The current value of the warning switch. $WARNING $^X The name of the Perl binary executable. $EXECUTABLE_NAME $ARGV The current filename when reading from the default file handle. ### Global Array Special Variables @ARGV The list of command-line arguments passed to the script. @INC The list of directories to search when importing modules. @F The array input from the command line. ### Global Hash Special Variables %INC The hash %INC contains all files included via do or require statements. The key is the filename, and the value is the path to that file. %ENV Contains the current environment variables. %SIG The list of signals and their handling methods. ### Global Special File Handles ARGV A special file handle that iterates through all filenames in the array variable @ARGV. STDERR Standard error output handle. STDIN Standard input handle. STDOUT Standard output handle. DATA A special file handle that references anything after the __END__ marker in the file, including script content. Or it references all content after the __DATA__ marker in a file, as long as you are reading data in the same package, __DATA__ exists. _ (Underscore) A special file handle used to cache file information (fstat, stat, and lstat). ### Global Special Constants __END__ The logical end of the script; text after this is ignored. __FILE__ The current filename. __LINE__ The current line number. __PACKAGE__ The current package name; the default package name is main. ### Regular Expression Special Variables $n Contains the nth substring from the last pattern match. $& The string from the last successful pattern match. $MATCH $` The content before the substring matched in the last successful match. $PREMATCH $' The content after the substring matched in the last successful match. $POSTMATCH $+ The last bracket matched by the last successful regular expression search pattern. For example: /Version: (.*)|Revision: (.*)/ && ($rev = $+); $LAST_PAREN_MATCH ### File Handle Special Variables $| If set to zero, after each call to write or print, the fflush function is automatically called to write the content back to the file. $OUTPUT_AUTOFLUSH $% The current output page number. $FORMAT_PAGE_NUMBER $= The current page length. Default is 60. $FORMAT_LINES_PER_PAGE $- The number of lines remaining on the current page. $FORMAT_LINES_LEFT $~ The name of the current report output format. The default value is the file handle name. $FORMAT_NAME $^ The name of the current report output header format. The default value is the file handle name with the suffix "_TOP". $FORMAT_TOP_NAME