Learning Perl, Fourth Edition

Notes on Lexical (my) Variables

Those lexical variables can be used in any block, not merely in a subroutine’s block. For example, they can be used in the block of an if, while, or foreach:

    foreach (1..10) {
      my($square) = $_ * $_;  # private variable in this loop
      print "$_ squared is $square.\n";
    }

The variable $square is private to the enclosing block; in this case, that’s the block of the foreach loop. If there’s no enclosing block, the variable is private to the entire source file. For now, your programs aren’t going to use more than one source file, so this isn’t an issue. But the important concept is that the scope of a lexical variable’s name is limited to the smallest enclosing block or file. The only code that can say $square and mean that variable is the code inside that textual scope. This is a big win for maintainability—if the wrong value is found in $square, the culprit will be found within a limited amount of source code. As experienced programmers have learned (often the hard way), limiting the scope of a variable to a page of code, or to a few lines of code, accelerates the development and testing cycle.

Also, the my operator doesn’t change the context of an assignment:

    my($num) = @_;  # list context, same as ($num) = @_;
    my $num  = @_;  # scalar context, same as $num = @_;

In the first one, $num gets the first parameter, as a list-context assignment; in the second, it gets the number of parameters, in a scalar context. Either line of code could be what the programmer wanted; ...