Why can't I use the value of a Perl variable to access the name of a lexical variable?

Question

Why is this seal 42:

$answer = 42; $variable = "answer"; print ${$variable} . "\n"; 

but this is not so:

 my $answer = 42; my $variable = "answer"; print ${$variable} . "\n"; 

Answer 1

Only package variables (the view indicated in your first example) can be targeted using symbolic links. There can be no lexical ( my ) variables, so your second example does not work.

See the excellent “Copy with Scoping” article on how two separate system variables work in Perl. And see. Also excellent. Why is it stupid to use the variable-name for why it's stupid. :)

Answer 2

Perl has two completely separate, but largely compatible variable systems, package variables, as in the first example, and lexical variables, as in the second. There are a few things that everyone can do, besides the other:

Package variables are the only ones that can be:

• localized (with local )
• used as the target for a symbolic link (the reason the second OP example is not working)
• used as simple words (auxiliary definitions, file descriptors)
• used with typeglobs (because this is what the character really is under the hood)

Lexical variables are the only ones that can be closed (used in lexical closure).

Using a string value will help you declare package variables with our , making the difference more clear.

There are several times when symbolic links are useful in Perl, most of which are centered around manipulating a symbol table (for example, writing your own import in a module, rather than using Exporter , monkey-fixing modules at run time, -programming tasks). These are all advanced topics.

For other tasks, there is usually a better way to do this, for example with a hash. The general rule is to always work under use warnings; use strict; use warnings; use strict; if you do not know that there is no other way but to disable a part of the pragma (for example, using no strict 'refs'; as small a scope as possible).

Answer 3

Symbolic links only work with package variables. The symbol table does not track lexical variables (this is all vocabulary :).

Answer 4

The problem is that you cannot use a symbolic link to refer to a lexical variable. In both examples, ${$variable} searches for $main::answer . In the first case, $answer is a global package and shortened for $main::answer , so the link finds it. In the second, $answer is a lexical variable and does not live in any package, so the link cannot find it.

More in perlref under the heading Symbolic links .