I wrote an app almost 10 years ago . Just looked through the code a bit. It says "Build with PRC-Tools 2.0 and Palm OS SDK 4.0 or better". Written all in C++. Most of my work I did in a simple text editor. Not in an IDE. And I used makefiles I wrote myself and a command-line to compile the lot. I remember compiling for the m68k mostly because that would cover all our potential customers. But there was already ARM support in the works... until we just dropped it. We had too few sales to warrant further development. Not that our version for WinCE PDAs was doing any better, by the way. A couple of years later the first IPhone was released. And for that platform we still develop and sell.
There was partial support for C++ in 'recent' versions of PRC tools... classes and use of new and delete, and if you really wanted them, exceptions and RTTI. But no STL, streams... I wrote a file io and string class to keep the back-end code compatible (we had a significant chunk of back-end code already written in C/C++ and used it in apps for many platforms).
Also you had to manually segment your code for Palm OS because there was a fixed code segment size (my app used five code segments and I had to specify for each function in which code ). Long function calls were used to make the segments interact and you had to keep in mind to use them as little as possible because, although the compiler did the work of adding those long function calls for you, they still incurred a performance hit.
I finally retired my company Palm M515 two years ago when I got a second hand HTC HD2 from a friend (and promptly updated it with CyanogenMod). I managed to save most of my (calender, notes, contacts) data and it's now part of the Google cloud...
It was fun to program for that platform, I learned a lot from it but I would say, in this day and age, it's utterly useless to start a new project for it. I would recommend Android over iOS, if you just want to make a mobile app. Although Apple's tools (XCode) are better IMHO, you can do a lot more with Android and the apps you make for them before you have to spend money on it (AFAIK the only thing Google asks is a $25 one-time fee if you want to publish on the play store and you can always side-load for 'homebrew'). And you can develop and compile an Android app on a variety of computer hard- and software platforms, while a recent Apple (i)Mac(book/mini) is required to make iOS apps.
If you're really adamant to create a PalmOS app, get yourself PRC-tools, cygwin and a 'recent' PalmOS SDK and you might actually manage to get a working app. Time-stamps of my versions are of april 2004 so I do not have the final versions... Also, the PalmOS 4 emulator and/or PalmOS 5 simulator, with roms and of course the full Palm desktop suite installed on your machine so you can make backups and install
Ow, yes, there is a PalmOS developer suite for PalmOS 5 and 6, using Eclipse and a lot of integrated goodies. A 250MB sized beastie... but I haven't got the faintest clue where to get it nowadays.