The most common reason evaps perforate is that moisture causes the system to corrode from the outside in. The corrosion occurs because as warm (ambient) air is passed over the cooled coils, the moisture in the air will condense onto the coils. And often, people will run their AC until their destination, thus leaving the coils wet. When dirty air is passed over wet coils, the dirt will adhere to the coils, making it even more difficult for the water on the coils to evaporate. Thus filtering the air of this dirt will help to lengthen the life of the evap. And this is why EVERYONE in the know, recommends running the HVAC fan with the AC off for the last mile or so of each trip in order to dry the evap off.
 

Just had to tell someone that I replaced my own evap core last week.  Took about 20
manhours for a very experienced shade tree mechanic.  Mostly just had to take time to
bag up and label all parts (screws & fasteners) as well as writing down each step and how
long it took.  This was the most difficult repair I have ever done - and I have rebuilt engines
before!  However the air is now ice cold, though it has only been a week...

BTW, I bought the parts from Swedish Engineering.  Great guys, they give good advice
and even faxed me the instructions for the repair out of their copy of the official Volvo
service manual.  Also, their price was fantastic, about $320 for the complete evap kit.

So, unless you consider yourself an expert home mechanic, I agree with everyone's
assessment, let the pros handle this one.  But if you are bold - well I wish you much
success and very cold air!

Jon Baugh