in reply to $_ vs. argument passing

One of the reasons why the arg version is so much faster is because of a little-known feature of Perl: the arguments to functions are passed by-reference. That means when you use $_[0], you're actually referring to the constant 'foobar' which is embedded in the Perl bytecode.

On the other hand, your $_ = 'foobar' induces a string copy of 'foobar' every single time that statement is executed. Since calculating the length is easy, the additional overhead of the extra copying *really* stands out.

I added these two subs to the benchmark: 

sub xarg       { my $tmp = $_[0]; length $tmp }
sub xarg_shift { my $tmp = shift; length $tmp }

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And reran it (dropping the time from 60s each to 10s each, because I'm far too impatient to wait six minutes for the test to rerun): 
Benchmark: running arg, arg_shift, noarg, noarg2, xarg, xarg_shift for
+ at least 10 CPU seconds...
       arg: 10 wallclock secs (10.52 usr + -0.01 sys = 10.51 CPU) @ 48
+9653.09/s (n=5146254)
 arg_shift: 10 wallclock secs (10.28 usr +  0.00 sys = 10.28 CPU) @ 41
+4663.33/s (n=4262739)
     noarg:  9 wallclock secs (10.00 usr +  0.00 sys = 10.00 CPU) @ 28
+7455.40/s (n=2874554)
    noarg2: 11 wallclock secs (10.55 usr +  0.00 sys = 10.55 CPU) @ 28
+2535.73/s (n=2980752)
      xarg: 10 wallclock secs (10.59 usr +  0.00 sys = 10.59 CPU) @ 33
+7761.28/s (n=3576892)
xarg_shift: 10 wallclock secs (10.39 usr +  0.01 sys = 10.40 CPU) @ 29
+7917.40/s (n=3098341)
               Rate    noarg2     noarg xarg_shift      xarg arg_shift
+       arg
noarg2     282536/s        --       -2%        -5%      -16%      -32%
+      -42%
noarg      287455/s        2%        --        -4%      -15%      -31%
+      -41%
xarg_shift 297917/s        5%        4%         --      -12%      -28%
+      -39%
xarg       337761/s       20%       18%        13%        --      -19%
+      -31%
arg_shift  414663/s       47%       44%        39%       23%        --
+      -15%
arg        489653/s       73%       70%        64%       45%       18%
+        --

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Note that they're still not on equal footing: the additional symbol table lookup needed to find the location of $_ *really* hurts. This is because the work needed to find $_ in the symbol table is far greater than the work needed to find the length of it (because the length is precomputed).
But if you're worried about performance, keep in mind that your slowest one executed in an average of 5.5 microseconds, which on a short-running script isn't terrible.
--Stevie-O 
$"=$,,$_=q>|\p4<6 8p<M/_|<('=>
.q>.<4-KI<l|2$<6%s!<qn#F<>;$,
.=pack'N*',"@{[unpack'C*',$_]
}"for split/</;$_=$,,y[A-Z a-z]
         {}cd;print lc

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In Section Seekers of Perl Wisdom