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$, % and @?
It helps to equate these symbols to English words. The dollar symbol,
$, is like the word ``that''. It is singular. In Perl, the
$ symbol represents a ``scalar'' value. For now, let us just
say that $ means atomic and singularity, not a collection
of something.
There are two built-in collection types in Perl. The simpler one is the ``list'' type, and other one is the ``hash'' type. A list of items is nothing more than a sequence of items, in which the order is significant. A hash of items is like a phone book of items so that each item has a ``name'' (key) to uniquely identify itself from all the other items.
The % symbol is similar to the phrase ``that phone book of ...''
In other words, %ENV is ``that phone book of environment
variables''. The @ symbol is similar to the phrase
``that list of ...'' So, @envkeys is ``that list of
names of environment variables''.
Now, how come $ENV{$key} is right, and %ENV{$key} is
incorrect?
Let's use our informal English translation. %ENV{$key} is
roughly ``the value of that phone book of environment variables
with the name same as the value of $key.'' This makes no
sense! There is at most one single definition in the phone book with
the name identical to the value of $key.
$ENV{$key}, on the other hand, means ``the value of that
environment variable with the name identical to the value of
$key.'' This makes a lot more sense.
Note that in Perl, a variable must be preceded by one of
$, % or @.
Copyright © 2008-05-09 by Tak Auyeung