Here's a simplified test case. For added fun, it also works with @, $ and *:

#!perl
s//%/e;
}
[download]

Eval is actually trying to use the surrounding code to try and finish it's evaluation. This was fixed. Recent version of perl (e.g. 5.6) will give this warning when the code above is run:

Bad evalled substitution pattern at file.pl line X
[download]

Looking at perl5004delta, we read that:

Perl (version 5) used to determine the value of 
EXPR inconsistently, sometimes incorrectly using 
the surrounding context for the determination. Now, 
the value of EXPR (before being parsed by eval) is 
always determined in a scalar context.
[download]

Unfortunately, each version keeps removing some of these little "features" that make for great obfuscated code. :)

Here's some fun code to try out if you have an "older" version of perl (older is in quotes because it hasn't been fixed *that* recently):

#!perl

$foo = "ABC";
print "($foo)\n";
$foo =~ s/B/*/e;
}
print "($foo)\n";
[download]

Running this script produces this rather odd output:

(ABC)
(A*main::}
C)
[download]

In other words, eval tried so hard to evaluate something, it regarded the * as a typeglob, and created something unusual, to say the least. :)