Answer.

Background:

In Perl, both global and local variables have been actively used since the beginning. Later, lexical variables were introduced with the my keyword. This provided developers with convenient tools for controlling variable scope and preventing name conflicts.

Issue:

Errors often arise from confusion between global (package variables) and lexical variables, incorrect use of local and my, as well as overriding global values during code execution. In large projects, careless management of scopes can lead to hard-to-trace bugs.

Solution:

Use my to declare variables with lexical scope (scoped within the block where they are declared), and global variables only when necessary. For temporarily replacing the value of a global variable, use local, which saves the original value until the end of the block. For package variables — our. A correct understanding of these differences helps avoid side effects.

Example code:

our $global = 10;
sub demo {
    my $lexical = 20;
    local $global = 99; # temporary change of global
    print "Inside demo: $global, $lexical
";
}
demo();
print "Outside demo: $global
";

Key features:

my creates variables visible only within the current block
local temporarily changes global variables
our declares package variables for use outside the current file

Tricky questions.

Can a variable declared with my be accessible outside the current block?

No. The lexical variable my is only visible within the block where it was created; it does not exist beyond that.

What is the difference between local and our?

local temporarily changes the value of a global variable for the duration of the block, while our is used to declare a variable visible throughout the package and does not create a copy of the value.

Example code:

our $var = 1; # global package variable
sub test {
    local $var = 3; # temporarily overrides $var to 3
    print $var;
}

Can my be used inside eval to make the variable visible outside eval?

No. The scope of variables declared with my inside eval is limited to that eval block only.

Common mistakes and anti-patterns

Declaring variables with the same names in different scopes
Using global variables unnecessarily
Confusion between my and local, misunderstanding the behavior of these keywords

Real-life example

** Negative case

In a large Perl project, global variables were used everywhere without explicit scope indication (without my and our). Once, a new developer accidentally overridden one of those variables in a module, leading to unpredictable results in production.

Pros:

Variables are accessible everywhere

Cons:

Hard to trace the cause of bugs
Difficult code maintenance
Possible unexpected side effects

** Positive case

In a new project, all variables were declared using my within functions and blocks, and globals were declared with our only as needed, with clear documentation.