To pass data via a URL you would use something like this:

http://not.dave.org.uk/cgi-bin/script.pl?key1=val1&key2=val2
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The parameters are appended to the end of the URL following a question mark (?). Parameters are arranged in key/value pairs where key and value are separated by an equals sign (=). Each pair is spearated from the next with an ampersand (&). Note that certain characters (including ?, = and & for obvious reasons) cannot be used in a URL and must therefore be encoded.

Within the CGI program, extracting the parameters is very easy as long as you use CGI.pm. Not using CGI.pm is dangerous as CGI parameter extraction is a tricky business and very easy to get wrong. Using CGI.pm, you can just write code like this:

#!/usr/bin/perl -w
use strict;
use CGI qw(:standard);

print header(-type => 'text/plain');

foreach (param) {
  print "Parameter $_ is ", param($), "\n";
}
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Note that it is perfectly legal to have multi-valued CGI parameters, like this:

key1=val1&key2=val2a&key2=val2b
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In this case the param function will return a list when asked for parameter key2. In this case the sample code above will break.

Get more info on this from perldoc CGI or Lincoln Stein's book The Official Guide to Programming with CGI.pm.