In Perl 4 there were two ways to define
the package scope of a variable. First, the variable might have
Global scope means that the variable could
be seen (accessible/modifiable) by every other object in the program.
Sometimes this is a good thing. Sometimes you have variables,
for example, whose values are relevant to every aspect of your program.
However, most times, it is better to limit the
scope of variables to specific areas in your program. Limiting the
scope helps maintain name space, reduce programmatic complexity,
and increases modularity of design.
To limit the scope of a variable, Perl initially provided
the "local" keyword. The local keyword was used to limit the scope
of a variable to within its enclosing brackets
Consider the following example. Notice that when
the local keyword is applied to the variable $a, changes made to
$a inside of the subroutine have no affect on the value of $a outside
of the subroutine. On the other hand, $b is not declared local in the
subroutine and hence, changes made to $b in the subroutine affect $b
outside of the subroutine:
In Perl 5, localization is handled by the "my"
keyword which hides the variable from the outside world completely.
Hey, isn't that what local was supposed to do?
Well, not exactly. The local keyword is meant to
assure "dynamic" scoping whereas the "my" keyword is for "lexical" scoping.
The big difference is that dynamically scoped
variables are visible not only from within the enclosing brackets
but in all subroutines called within those brackets. Lexical variables
are totally hidden. Consider the following examples. Notice in particular that when
"my" is applied to $a, the second subroutine cannot see the change whereas when "local"
is applied, it can.
Another aspect of localizing Perl 5 variables is understanding how
to make a variable local within a package or file but not within the scope
of the whole program. In small programs, global variables can be much easier
to deal with instead of having to recall and type the
calling conventions of every defined
Of course, we recommend local variables for the reasons stated
earlier, but in those cases where strict adherence to local variables is
painful, Perl supports a kind of compromise between global and local
variables. Perl supports the concept of package-level and file-level
Using the "my" keyword in a file outside
the context of a subroutine will do this.
Likewise if you wish to declare a variable that is local to a package, you
can use the "use vars" pragma as shown below:
use vars qw($package_global_variable);
One caveat to using my to declare global variables is that you should be
careful if you use them in conjunction with scoping tricks. In particular,
one complex trick is called a "closure". Closure's are beyond the scope of this
document, you can find it by using the perldoc command to read the perlfaq7 or the
perlref document using the following syntax on your command-line (DOS for Windows,
shell for UNIX users):
Although local() is a Perl 4 keyword and in many cases, the my keyword is
better for localizing variables in a manner that most programmers expect,
local() does still have some uses in Perl 5. Talking about these uses is
beyond the scope of this document but you can get more information about
localization using the local keyword
from the MJD article at
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