Imported symbols act just like global variables; they can add up memory quickly. In addition to polluting the namespace, a process grows by the size of the space allocated for all the symbols it imports. The more you import (e.g., qw(:standard) versus qw(:all) with, the more memory will be used.

Let's say the overhead is of size Overhead. Now take the number of scripts in which you deploy the function method interface—let's call that Scripts. Finally, let's say that you have a number of processes equal to Processes.

You will need Overhead × Scripts × Processes of additional memory. Taking an insignificant Overhead of 10 KB and, adding in 10 Scripts used across 30 Processes, we get 10 KB × 10 × 30 = 3 MB! The 10-KB overhead becomes a very significant one.

Let's assume that we need to use strtol( ) from the POSIX package. Under Perl 5.6.1 we get:

panic% ./ 'use POSIX ( ); POSIX::strtol(_ _PACKAGE_ _, 16)'
use POSIX ( ) added  176k

panic% ./ 'use POSIX; strtol(_ _PACKAGE_ _, 16)'
use POSIX added  712k

The first time we import no symbols, and the second time we import all the default symbols from POSIX. The difference is 536 KB worth of aliases. Now let's say 10 different Apache::Registryscripts 'use POSIX;' for strftime( ), and we have 30 mod_perl processes:

536KB  ×  10  ×  30 = 160MB

We have 160 MB of extra memory used. Of course, you may want to import only needed symbols:

panic% ./ 'use POSIX qw(strtol); strtol(_ _PACKAGE_ _, 16);'
use POSIX qw(strftime) added  344k

Still, using strftime( ) uses 168 KB more memory. Granted, POSIX is an extreme case—usually the overhead is much smaller for a single script but becomes significant if it occurs in many scripts executed by many processes.

Here is another example, now using the widely deployed module. Let's compare's object-oriented and procedural interfaces. We'll use two scripts that generate the same output, the first (Example 13-13) using methods and the second (Example 13-14) using functions. The second script imports a few functions that are going to be used.

Example 13-13.

use CGI ( );
my $q = CGI->new;
print $q->header;
print $q->b("Hello");

Example 13-14.

use CGI qw(header b);
print header( );
print b("Hello");

After executing each script in single server mode (-X), we can see the results with the help of Apache::Status, as explained in Chapter 9.

Here are the results of the first script:

Totals: 1966 bytes | 27 OPs

handler 1514 bytes | 27 OPs
exit     116 bytes |  0 OPs

The results of the second script are:

Totals: 4710 bytes | 19 OPs

handler  1117 bytes | 19 OPs
basefont  120 bytes |  0 OPs
frameset  120 bytes |  0 OPs
caption   119 bytes |  0 OPs
applet    118 bytes |  0 OPs
script    118 bytes |  0 OPs
ilayer    118 bytes |  0 OPs
header    118 bytes |  0 OPs
strike    118 bytes |  0 OPs
layer     117 bytes |  0 OPs
table     117 bytes |  0 OPs
frame     117 bytes |  0 OPs
style     117 bytes |  0 OPs
Param     117 bytes |  0 OPs
small     117 bytes |  0 OPs
embed     117 bytes |  0 OPs
font      116 bytes |  0 OPs
span      116 bytes |  0 OPs
exit      116 bytes |  0 OPs
big       115 bytes |  0 OPs
div       115 bytes |  0 OPs
sup       115 bytes |  0 OPs
Sub       115 bytes |  0 OPs
TR        114 bytes |  0 OPs
td        114 bytes |  0 OPs
Tr        114 bytes |  0 OPs
th        114 bytes |  0 OPs
b         113 bytes |  0 OPs

As you see, the object-oriented script uses about 2 KB of memory while the procedural interface script uses about 5 KB.

Note that the above is correct if you didn't precompile all of's methods at server startup. If you did, the procedural interface in the second test will take up to 18 KB, not 5 KB. That's because the entire namespace is inherited, and it already has all its methods compiled, so it doesn't really matter whether you attempt to import only the symbols that you need. So if you have:

use CGI  qw(-compile :all);

in the server startup script, having:

use CGI qw(header);


use CGI qw(:all);

is essentially the same. All the symbols precompiled at startup will be imported, even if you request only one symbol. It seems like a bug, but it's just how works.