Variables (Programming Perl)

2.4. Variables

Not surprisingly, there are three variable types corresponding to the three abstract data types we mentioned earlier. Each of these is prefixed by what we call a funny character.[5] Scalar variables are always named with an initial $, even when referring to a scalar that is part of an array or hash. It works a bit like the English word "the". Thus, we have:

[5] That's another technical term in computer science. (And if it wasn't before, it is now.)

Construct	Meaning
`$days`	Simple scalar value `$days`
`$days[28]`	29th element of array `@days`
`$days{'Feb'}`	"`Feb`" value from hash `%days`

Note that we can use the same name for $days, @days, and %days without Perl getting confused.

There are other, fancier scalar terms, useful in specialized situations that we won't go into yet. They look like this:

Construct	Meaning
`${days}`	Same as `$days` but unambiguous before alphanumerics
`$Dog::days`	Different `$days` variable, in the `Dog` package
`$#days`	Last index of array `@days`
`$days->[28]`	29th element of array pointed to by reference `$days`
`$days[0][2]`	Multidimensional array
`$days{2000}{'Feb'}`	Multidimensional hash
`$days{2000,'Feb'}`	Multidimensional hash emulation

Entire arrays (or slices of arrays and hashes) are named with the funny character @, which works much like the words "these" or "those":

Construct	Meaning
`@days`	Array containing `($days[0], $days[1],... $days[n])`
`@days[3, 4, 5]`	Array slice containing `($days[3], $days[4], $days[5])`
`@days[3..5]`	Array slice containing `($days[3], $days[4], $days[5])`
`@days{'Jan','Feb'}`	Hash slice containing `($days{'Jan'},$days{'Feb'})`

Entire hashes are named by %:

Construct	Meaning
`%days`	`(Jan => 31, Feb => $leap ? 29 : 28, ...)`

Any of these constructs may also serve as an lvalue, specifying a location you could assign a value to. With arrays, hashes, and slices of arrays or hashes, the lvalue provides multiple locations to assign to, so you can assign multiple values to them all at once:

@days = 1 .. 7;