What is the difference between my and local in Perl?

Question

I am seeing both of them used in this script I am trying to debug and the literature is just not clear. Can someone demystify this for me?

Answer 1

"my" variables are visible in the current code block only. "local" variables are also visible where ever they were visible before. For example, if you say "my $x;" and call a sub-function, it cannot see that variable $x. But if you say "local $/;" (to null out the value of the record separator) then you change the way reading from files works in any functions you call.

In practice, you almost always want "my", not "local".

Answer 2

http://perldoc.perl.org/perlsub.html#Private-Variables-via-my()

Unlike dynamic variables created by the local operator, lexical variables declared with my are totally hidden from the outside world, including any called subroutines. This is true if it's the same subroutine called from itself or elsewhere--every call gets its own copy.

http://perldoc.perl.org/perlsub.html#Temporary-Values-via-local()

A local modifies its listed variables to be "local" to the enclosing block, eval, or do FILE --and to any subroutine called from within that block. A local just gives temporary values to global (meaning package) variables. It does not create a local variable. This is known as dynamic scoping. Lexical scoping is done with my, which works more like C's auto declarations.

I don't think this is at all unclear, other than to say that by "local to the enclosing block", what it means is that the original value is restored when the block is exited.

Answer 3

local is an older method of localization, from the times when Perl had only dynamic scoping. Lexical scoping is much more natural for the programmer and much safer in many situations. my variables belong to the scope (block, package, or file) in which they are declared.

local variables instead actually belong to a global namespace. If you refer to a variable $x with local, you are actually referring to $main::x, which is a global variable. Contrary to what it's name implies, all local does is push a new value onto a stack of values for $main::x until the end of this block, at which time the old value will be restored. That's a useful feature in and of itself, but it's not a good way to have local variables for a host of reasons (think what happens when you have threads! and think what happens when you call a routine that genuinely wants to use a global that you have localized!). However, it was the only way to have variables that looked like local variables back in the bad old days before Perl 5. We're still stuck with it.

Answer 4

I can’t believe no one has linked to Mark Jason Dominus’ exhaustive treatises on the matter:

• Coping with Scoping

• And afterwards, if you want to know what local is good for after all,
  Seven Useful Uses of local

Answer 5

Your confusion is understandable. Lexical scoping is fairly easy to understand but dynamic scoping is an unusual concept. The situation is made worse by the names my and local being somewhat inaccurate (or at least unintuitive) for historical reasons.

my declares a lexical variable -- one that is visible from the point of declaration until the end of the enclosing block (or file). It is completely independent from any other variables with the same name in the rest of the program. It is private to that block.

local, on the other hand, declares a temporary change to the value of a global variable. The change ends at the end of the enclosing scope, but the variable -- being global -- is visible anywhere in the program.

As a rule of thumb, use my to declare your own variables and local to control the impact of changes to Perl's built-in variables.

For a more thorough description see Mark Jason Dominus' article Coping with Scoping.

Answer 6

dinomite's example of using local to redefine the record delimiter is the only time I have ran across in a lot of perl programming. I live in a niche perl environment [security programming], but it really is a rarely used scope in my experience.