By folding the suggestions encountered so far, and adding in some of my own, I was able to bring the total down to 342 characters. Big gains can be made by observing that $foo[-2] refers to indices at the end of an array. That happens twice in this script.

In the part where the code pops to $_ and $0, it's cheaper to just call pop @n directly. This was the case with pop@n, and I guess it will still hold with p.

sub w as written contains a bug. Sometimes you push more than one element on. But that's okay, @_ is a win over $_[0].

I tried changing expr if cond to cond&&expr but that didn't do anything.

map{push@L,@C+0;push@C,lc=~/./g}<>;sub p{pop@n}sub w{push@n,@_}while($
+n<@C){$_=
$C[$n++];if(/n/){$0=0;/-/ or$0=$_+$0*7while($_=index"htaoinse",$C[$n++
+])<7;w$0}
if(/t/){$_=p;$_?$n=$L[$_-1]:last if p}/a/&&w 1+grep{$n>$_}@L;if(/h/){$
+_=p;w$n[-
abs$_];$_>0&&splice@n,-1-$_,1}/o/&&print chr p;/s/&&w-p()+p;/e/&&w int
+ p()/p,p%
p;w($_=getc)?ord:-1if/i/}
[download]

Note that changing /s/&&v-(p)+p to /s/&&v+p-p breaks the interpreter. Or at least, the hello.eta program no longer runs correctly, which was the extent of my testing. The interpreter is quite slow.