How about Arc support in Vim? I seem to recall reading that Paul Graham uses Vim, so I imagine he probably has some tools specific to Arc using Vim in his developer toolbox.
Why not write an Arc interpreter in Perl -- or even an interpreter for a web-centric subset of Arc? Stick the interpreter in the cgi-bin directory and use that to run your Arc scripts. Perl/CGI is even more ubiquitous on shared hosting accounts than PHP, supports closures, et cetera.
In fact, I've considered creating a new closures-based object system in Perl 5 in the past (never got around to it), which sounds suspiciously like it might have some things in common with some characteristics of Arc.
