Programming Perl, 3rd Edition

Symbolic References

What happens if you try to dereference a value that is not a hard reference? The value is then treated as a symbolic reference. That is, the reference is interpreted as a string representing the name of a global variable.

Here is how this works:

$name = "bam";
$$name = 1;                # Sets $bam
$name->[0] = 4;            # Sets the first element of @bam
$name->{X} = "Y";          # Sets the X element of %bam to Y
@$name = ();               # Clears @bam
keys %$name;               # Yields the keys of %bam
&$name;                    # Calls &bam

This is very powerful, and slightly dangerous, in that it's possible to intend (with the utmost sincerity) to use a hard reference, but to accidentally use a symbolic reference instead. To protect against that, you can say:

use strict 'refs';

and then only hard references will be allowed for the rest of the enclosing block. An inner block may countermand the decree with:

no strict 'refs';

It is also important to understand the difference between the following two lines of code:

${identifier};    # Same as $identifier.
${"identifier"};  # Also $identifier, but a symbolic reference.

Because the second form is quoted, it is treated as a symbolic reference and will generate an error if use strict 'refs' is in effect. Even if strict 'refs' is not in effect, it can only refer to a package variable. But the first form is identical to the unbracketed form, and will refer to even a lexically scoped variable if one is declared. The next example shows this (and the next section discusses it).

Only package variables are ...