my operator can also be used at the outermost level of your program, outside of any subroutines or blocks. While
my isn't really a local variable in the sense defined above, it's actually rather useful, especially when used in conjunction with a Perl pragma:
 A pragma is a compiler directive. Other directives include those used to set up integer arithmetic, overload numeric operators, or request more verbose warnings and error messages. These are documented in perlmodlib.
If you place this pragma at the beginning of your file, you will no longer be able to use variables (scalars, arrays, and hashes) until you have first declared them. And you declare them with
my, like so:
use strict; my $a; # starts as undef my @b = qw(fred barney betty); # give initial value ... push @b, qw(wilma); # cannot leave her out @c = sort @b; # WILL NOT COMPILE
That last statement will be flagged at compile time as an error, because it referred to a variable that had not previously been declared with
my (that is,
@c). In other words, your program won't even start running unless every single variable being used has been declared.
The advantages of forcing variable declarations are twofold:
Your programs will run slightly faster (variables created with
my are accessed slightly faster than ordinary variables).
 In this case, ordinary variable is really a package variable (so
$main::x). Variables created with
myare not found in any package.
Because of these advantages, many Perl programmers automatically begin every new Perl program with