Brothers, I wish to tread as lightly here as the beating of a butterfly's wings....and try not to start a Tidal Wave.
This is, after all, Flame Bait ..
Lest not we start a(n) (un)Holy War, I will say that truly IMHO either approach is valid. However, that was not your question. Your question was how to make a case for PERL.
First of all it is correct to point out that IBM's WebSphere, etc. is amongst the poorer choices one might make for a Java Application Server. In my experience they'd be better off buying into ATG Dynamo or at worst BEA Weblogic. So right there, it shows that they have not done their homework. That should cast doubt immediately on their plans.
Secondly, I'd like to state that there is no disparity of toolkits available in the Java realm, however... And this is an important point.. You have to pay for them. Whereas PERL is free to all who care to learn. That reduces the project overhead to a manhours driven budget. Much simpler.
Third, if you dare, slip them the URL for perlmonks on a piece of paper (ack! Dead Trees! Better to beam them on their Palm. Electrons aren't clearpathed..) and show them the wide availability of resources available. (Not to mention your monstrous XP. ;-))
Wait! This isn't a Parachute, this is a Backpack!