Why is perl so frequently used for automation?

Is it? I thought the meme of the last dozen or so years was "Perl is dead". It will depend on who you ask. Some will say Perl is used more often that it should -- others will say it's hard from them to find a Perl job, and that hence, Perl isn't used frequently enough (of course, the reason why they can't find a job maybe unrelated to how often Perl is used).

But how many big companies are there that do a significantly amount of their IT business in Perl? I don't think there are many. Let's multiply the revenue of a company with the fraction of their IT work done in Perl (to not have the amounts dwindle to nothing, only consider their in-house produce, and what was tailor made externally). For how many companies will this product reach 1 billion USD/year? Not many, I'm afraid.

Why is perl so effective in automation?

Is it? I think Perl works well in some cases. It works less in others. It would be hard to come up with a scenario where Perl would be the clear winner against other solutions.