$foo = "CGI";
require $foo; #this will bork
$foo = "CGI.pm";
require $foo; #this won't

##</code><code>##

BEGIN {
    sub which_finds_modules {
        #stuff goes here
    }

    @modules = which_finds_modules;
    foreach $mod(@modules) {
        require $mod; #make sure it has the extension!
    }
}