I do not use Perl but in java you can use webdriver for firefox to use a custome profile via:
FirefoxProfile profile = new FirefoxProfile();
profile.setAcceptUntrustedCertificates(true);
profile.setAssumeUntrustedCertificateIssuer(true);
profile.setPreference("network.proxy.http_port", proxy.getPort());
profile.setPreference("network.proxy.type", 1);
profile.setPreference("network.proxy.share_proxy_settings", true);
capabilities.setCapability(FirefoxDriver.PROFILE, profile);
and adding that capability to firefox driver. I know that you can also set firefox to use a specific profile by supplying in the command line when you start the selenium node for firefox using "webdriver.firefox.profile" as part of -browser setting for firefox.
cheers,
Suren