I just took the time to do some testing on several machines. All these accepted to modify the value:

$ perl -le'sub mychop(\$) { chop ${shift()} } print mychop $_ foreach 
+(qw/foo/);'
o
$ perl -le'sub mychop(\$) { chop ${$_[0]} } print mychop $_ foreach (q
+w/foo/);'
o

This is perl, version 5.005_02 built for PA-RISC1.1 - HP-UX td192 B.11
+.11
This is perl, v5.8.3 built for i586-linux-thread-multi - SUSE LINUX En
+terprise Server 9
This is perl, v5.8.3 built for IA64.ARCHREV_0-thread-multi - HP-UX td1
+64 B.11.23
This is perl, v5.8.3 built for x86_64-linux-thread-multi - SUSE LINUX 
+Enterprise Server 9
This is perl, v5.8.4 built for alpha-linux-thread-multi - Debian/Linux
+ 3.1
This is perl, v5.8.4 built for i386-linux-thread-multi - Debian/Linux 
+3.1
This is perl, v5.8.5 built for i386-linux-thread-multi - Red Hat Enter
+prise Linux AS release 4
This is perl, v5.8.5 built for x86_64-linux-thread-multi - Red Hat Ent
+erprise Linux AS release 4
This is perl, v5.8.8 built for hppa-linux-gnu-thread-multi - Debian/Li
+nux 3.1
[download]

While these refused to:

> perl -le'sub mychop(\$) { chop ${shift()} } print mychop $_ foreach 
+(qw/foo/);'
Modification of a read-only value attempted at -e line 1.
> perl -le'sub mychop(\$) { chop ${$_[0]} } print mychop $_ foreach (q
+w/foo/);'
Modification of a read-only value attempted at -e line 1.

This is perl, v5.8.0 built for alpha-dec_osf - Compaq Tru64 UNIX V5.1B
This is perl, v5.8.0 built for i386-netbsd - NetBSD 3.0
This is perl, v5.8.6 built for alpha-freebsd - FreeBSD 6.0-RELEASE
This is perl, v5.8.6 built for i386-freebsd-64int - FreeBSD 5.4-RELEAS
+E
[download]

Update: I added the operating system by hand, it's not part of the perl -v output on any system.