For me, that means application software that is part of the company infrastructure.
I specifically mentioned 'application software' to separate it from the network and/or systems software that is crucial but doesn't really fall under the umbrella of 'enterprise' software. Perl, in and of itself is not 'enterprise' software, although there may be Perl applications that fall into that category.
I've skimmed over the other answers, and at least one refers to the 'expensive salesmen and consultants' part of Enterprise Software. Something like SAP or PeopleSoft comes to mind there. The whole 'enterprise' thing makes me squirm -- it makes me think of expensively suited and well-coiffed professionals sitting at glass conference tables pulling glossy brochures out of spanking new leather briefcases. Of course it's going to be expensive, they answer bluntly .. but how much is your business worth? As they gently pooh-pooh your home grown system built with open source tools.
It seems 'enterprise' means you're not smart enough to be able to build your own system, so you have to hire outside help to do that. Why reinvent the wheel, right? Well, reinventing the wheel can be cheaper in the long run if it's going to be 1/3 the cost of a consultant. And, at the end of the project, all the knowledge on how the system works will reside within the company, rather than inside someone's well-coiffed head. And anyway, that head is at some other client now and doesn't have time for you.
No, I am not a fan of 'enterprise' software as built by expensive consultants.
Alex / talexb / Toronto
"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds