A bug in your version of perl, perhaps? On mine, it dies:

sidhekin@blackbox:~$ perl -v

This is perl, v5.8.7 built for i486-linux

Copyright 1987-2005, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using `man perl' or `perldoc perl'.  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

sidhekin@blackbox:~$ perl
sub mychop(\$) {  chop ${shift()} }

foreach my $thing (qw/foo bar baz/) {
   print "$thing -> ";
   my $c = mychop $thing;
   print " $thing,$c\n";
}
__END__
Modification of a read-only value attempted at - line 1.
foo ->
[download]

Update: After some trial-and-error, it seems to be the case that, on threaded builds, trying to take a reference to an alias of a constant string creates a copy of that constant string and results in a reference to that copy, while on non-threaded builds, you get a reference to the constant itself.

# v5.8.3 built for i386-linux-thread-multi:
-bash-2.05b$ perl -le 'print map{chop$$_, $_}\$_, \$_, \$_ for "abc", 
+($x="abc")'
cSCALAR(0x804cbb8)cSCALAR(0x804cc78)cSCALAR(0x804c99c)
cSCALAR(0x805f69c)bSCALAR(0x805f69c)aSCALAR(0x805f69c)

# v5.8.7 built for i486-linux:
sidhekin@blackbox:~$ perl -le 'print map{chop$$_, $_}\$_, \$_, \$_ for
+ "abc", ($x="abc")'
Modification of a read-only value attempted at -e line 1.
Attempt to free unreferenced scalar: SV 0x812ec2c.
[download]

Whether this is a bug or undefined behaviour, though ... I'll leave that for others to decide.