Perl Special Variables

Perl defines some special variables, usually prefixed with $, @, or %, such as $_ .

Many special variables have a long English name; the operating system variable $! can be written as $OS_ERROR.

To use the English names of special variables, you need to add use English; at the top of your program. This allows you to use descriptive English special variables.

The most commonly used special variable is $_, which contains the default input and pattern matching content. Here is an example:

Example

#!/usr/bin/perl

foreach ('Google', 'tutorialpro', 'Taobao') {
    print $_;
    print "\n";
}

Executing the above program will output:

Google
tutorialpro
Taobao

In the following example, we do not use $_ to output content:

Example

#!/usr/bin/perl

foreach ('Google', 'tutorialpro', 'Taobao') {
    print;
    print "\n";
}

Executing the above program will output:

Google
tutorialpro
Taobao

In the example, "Google" is output first, followed by "tutorialpro", and finally "Taobao".

In the iteration loop, the current string is placed in $_, and then output via print. Additionally, print uses $_ by default when no output variable is specified.

Here are some places where Perl will assume 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", which defaults to STDIN.
• Various list functions, such as print() and unlink().
• Pattern matching operations "m//", "s///", and "tr///" without using the "=~" operator.
• The default iteration variable in a "foreach" loop when no other variable is given.
• The implicit iteration variable in grep() and map() functions.
• When a "while" loop has a single condition, and that condition tests the result of "", $_ is the default location for input records. This does not happen outside of "while" loop conditions. (Mnemonic: The underscore can be omitted in certain operations.)

Types of Special Variables

Special variables can be categorized based on their usage:

• 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 those with special characters and English forms:

| $_ | Default input and pattern matching content. | | $ARG | | $. | Current line number of the last file handle read. | | $NR | | $/ | Input record separator, default is newline. If undef, reads to end of file. | | $RS | | $, | Output field separator. | | $OFS | | $\ | Output record separator. | | $ORS | | $" | Similar to $, but used for interpolating arrays and slices in double-quoted strings. Default is a space. | | $LIST_SEPARATOR | | $; | Separator used in emulating multidimensional arrays. Default is "\034". | | $SUBSCRIPT_SEPARATOR | | $^L | 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 | Variable used to store formatted data before printing. | | $ACCUMULATOR | | $# | Default numeric output format for printing numbers (deprecated). | | $OFMT | | $? | Status of the last external command. | | $CHILD_ERROR | | $! | Numeric value is errno, string value is the corresponding system error string. | | $OS_ERROR or $ERRNO | | $@ | Error message from eval. If empty, the last eval succeeded. | | $EVAL_ERROR | | $$ | Process ID of the current Perl script. | | $PROCESS_ID or $PID | | $< | Real user ID of the current process. | | $REAL_USER_ID or $UID |

Special Variables in Perl

Global Array Special Variables

Global Hash Special Variables

Global Special File Handles

Global Special Constants

Regular Expression Special Variables

File Handle Special Variables