I believe (perhaps wrongly) that the first is a space for speed tradeoff
Except for edge cases and possible bugs, COW is intended on average to use less memory and less CPU.

The second is an (IMO) unnecessary fix for a non-problem
A non-problem that allows you to trivially DoS any web server where input from the client (such as headers or parameters) are fed into a perl hash.

Anyway, perl's hash handling has been getting faster, not slower in recent years. This trivial code (read 0.5M words from a dictionary file and store in a hash): 

open my $fh, "</usr/share/dict/words" or die;
my %h;
$h{$_}++ while <$fh>;

[download]
consumes the following number of CPU Mcycles under various perls: 
  5.8.9   1,245
  5.18.0  1,143
  5.20.0  1,113
  5.22.0  1,163
  5.24.0  1,089

[download]

Dave.

In reply to Re^4: Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why? by dave_the_m
in thread Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why? by Anonymous Monk

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