The thing that surprises me though is that trying to increment the hash keys doesn't die like this one:

I agree, it seems odd. Consider:

#! perl
use strict;
use warnings;
use Data::Dump;

my %h = (1 => 'Fred');
dd %h;
fn(%h);
dd %h;

sub fn
{
    print "BEGIN fn()\n";
    dd @_;
    $_[0]++;
    $_[1] = 'Barney';
    dd @_;
    print "-END- fn()\n";
}
[download]

Output:

12:02 >perl 541_SoPW.pl
(1, "Fred")
BEGIN fn()
(1, "Fred")
(2, "Barney")
-END- fn()
(1, "Barney")

12:02 >
[download]

The hash value is clearly aliased, since the assignment in sub fn persists in %h after the sub returns. But the hash key is not: it appears to be copied (passed by value), as $_[0] behaves as a variable local to sub fn. (Whereas a literal value like 42 or 'Wilma' is aliased, as evidenced by the Modification of a read-only value error message which results from trying to increment it or assign to it within the sub.)

An anomaly? Or a feature?