CGI.pm is a module included in the standard perl distribution. It is the most commonly used module for writing perl scripts that interact with the web.

I run across these mistakes a lot when reviewing beginner perl code. The first mistake has to do with the dual nature of perl modules. A perl module can be accessed either as a function-oriented module or as a object-oriented module or by both methods.

CGI.pm allows both. Under the hood, that makes for some messy looking code. In practice, we find a lot of bloated code. Normally, we would use either the function oriented or the OO version, not both.Here are a few of the most common errors I see.

PLAIN TEXT
PERL:
  1. #!/usr/bin/perl
  2.  
  3. use strict;
  4. use warnings;
  5.  
  6. use CGI qw( :standard );
  7.  
  8. my $q = CGI->new();

use CGI qw( :standard ); loads the standard subroutines which the author of CGI.pm deems necessary for interacting with and creating xhtml pages and forms. This means we are going to use the function-oriented interface to CGI.pm. my $q = CGI->new(); establishes we are going to use the object-oriented interface to CGI.pm.

There's nothing wrong with that, but I find most people making this mistake go on to ignore one of the interfaces. I think these beginners just don't understand what they are doing. How should this look? It depends on the interface we want to use.

Object-Oriented

PLAIN TEXT
PERL:
  1. use CGI;
  2.  
  3. my $q = CGI->new();

Function-Oriented

PLAIN TEXT
PERL:
  1. use CGI qw( :standard );

Actually, the last example is also a common mistake. When using the function-oriented interface to CGI.pm we are importing subroutines from a module into our main name space. Ideally we would like to import only those subroutines which we plan to use. Any other subroutines would "pollute" our namespace.

What's wrong with :standard? Chances are you won't use more than ten or twelve functions. :standard represents about a hundred functions. Some of those are very obscure functions. Some of them are not used in xhtml documents.

Here's a list from my version of CGI.pm. :standard is actually four other exported tags.

:html2:
h1                  h2                  h3                  h4
h5                  h6                  p                   br
hr                  ol                  ul                  li
dl                  dt                  dd                  menu
code                var                 strong              em
tt                  u                   i                   b
blockquote          pre                 img                 a
address             cite                samp                dfn
html                head                base                body
Link                nextid              title               meta
kbd                 start_html          end_html            input
Select              option              comment             charset
escapeHTML

:html3:
div                 table               caption             th
td                  TR                  Tr                  sup
Sub                 strike              applet              Param
embed               basefont            style               span
layer               ilayer              font                frameset
frame               script              small               big

:form:
textfield           textarea            filefield           password_field
hidden              checkbox            checkbox_group      submit
reset               defaults            radio_group         popup_menu
button              autoEscape          scrolling_list      image_button
start_form          end_form            startform           endform
start_multipart_form                    end_multipart_form  isindex
tmpFileName         uploadInfo          URL_ENCODED         MULTIPART

:cgi:
param               upload              path_info           path_translated
url                 self_url            script_name         cookie
Dump                raw_cookie          request_method      query_string
Accept              user_agent          remote_host         content_type
remote_addr         referer             server_name         server_software
server_port         server_protocol     virtual_host        remote_ident
auth_type           http                save_parameters     restore_parameters
param_fetch         remote_user         user_name           header
redirect            import_names        put                 Delete
Delete_all          url_param           cgi_error

I doubt anyone needs all of those. It is better to just import the ones you are using. I find this is all that is needed in most cases.

PLAIN TEXT
PERL:
  1. use CGI qw( param header *html );