Advanced Perl Programming

Symbolic References

Normally, a construct such as $$var indicates that $var is a reference variable, and the programmer expects this expression to return the value that was pointed to by $var when the references were taken.

What if $var is not a reference variable at all? Instead of complaining loudly, Perl checks to see whether $var contains a string. If so, it uses that string as a regular variable name and messes around with this variable! Consider the following:

$x = 10;
$var = "x";
$$var = 30;   # Modifies $x to 30 , because $var is a symbolic
              # reference !

When evaluating $$var, Perl first checks to see whether $var is a reference, which it is not; it’s a string. Perl then decides to give the expression one more chance: it treats $var’s contents as a variable identifier ($x). The example hence ends up modifying $x to 30.

It is important to note that symbolic references work only for global variables, not for those marked private using my.

Symbolic references work equally well for arrays and hashes also:

$var = "x";
@$var = (1, 2, 3);   # Sets @x to the enumerated list on the right

Note that the symbol used before $var dictates the type of variable to access: $$var is equivalent to $x, and @ $var is equivalent to saying @ x.

This facility is immensely useful, and, for those who have done this kind of thing before with earlier versions of Perl, is much more efficient than using eval. Let us say you want your script to process a command-line option such as "-Ddebug_level=3" and set ...