First off using subs or strings makes no difference to validity of the benchmark so long as everything benchmarked uses the same technique.
That's stupid. When you are benchmarking, you should minimize the time spend on doing things you are not interested in, otherwise it *does* muddy the waters. We are looking at ratios, not absolute differences, and a / b is usually different from (a + c) / (b + c) for c not equal to 0.

Calling Perl subroutines is not cheap.

Any difference that might result from that would be more than drowned out by noise and by the speed difference of the machines we are using to do the benchmarking.
What do you mean by that? We are comparing ratios. If the ratios of two algorithms would vary wildly from machine to machine, it would be utterly silly to publish any Benchmark. The point of looking at the ratios is to diminish the effect of the speed of the machine.
Why dont you run the code I posted and show us just how much it does cost on your system? On my system it would appear that it costs a 125% slowdown.
Right figure, wrong conclusion. The slowdown isn't mainly caused by the use of $&, but the copying involved. The blackadder program ends up with the server name in a normal variable - one which you can modify and won't be overwritten by the system. Not so by theorbtwo. By slightly modifying the code, assigning $1 to a variable, the difference drops sharply. Conclusion, blackadder is slower because there's more being copied - not because the use of $&. And that's not more than logical. Because $& isn't more costly than using parenthesis - the costs of $& and more so from $` and $' come if you are using other regural expression in your program, for which you don't use $& and friends. But that's not what you are benchmarking.

Here's my code, bm_blackadder.pl and bm_theorbtwo.pl as yours. 

# bm_theorbtwo_assign.pl
# benchmark saw ampersand -- theorbtwo
use strict;
use warnings;
use Benchmark qw(timethis);
use Data::Dumper;

my $count=$ARGV[0] || -1;
my $unc  =$ARGV[1] || '\\\\server_name\\sys_share';
print "Matching $unc for $count\n";
print Dumper(timethis($count,sub {
                  $unc =~ m/^\\\\([^\\]+)\\/;
                  my $server = $1;
                }, 'theorbtwo'
));
__END__  
# run_bm.pl
use Benchmark 'cmpthese';
use Data::Dumper;

sub run_bm($){
    my $str=shift;
    my $h;
    $str=~s/\A(.*)\$VAR1 =/$h=$1;''/se;
    print $h;
    my $v=eval($str);
    die $@ if $@;
    $v
}
   
my $opts='-5 \\\\\\\\foo\\\\bar\\\\baz.exe';

my $hash={
          blackadder => run_bm(`perl bm_blackadder.pl $opts`),
          theorbtwo  => run_bm(`perl bm_theorbtwo.pl $opts`),
          theorbtwo_assign  => run_bm(`perl bm_theorbtwo_assign.pl $op
+ts`),
         };
cmpthese($hash); 
__END__

Matching \\foo\bar\baz.exe for -5
blackadder:  4 wallclock secs ( 5.05 usr +  0.00 sys =  5.05 CPU) @ 71
+109.11/s (
n=359101)
Matching \\foo\bar\baz.exe for -5
 theorbtwo:  5 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 17
+1489.10/s
(n=896888)
Matching \\foo\bar\baz.exe for -5
 theorbtwo:  5 wallclock secs ( 5.31 usr +  0.00 sys =  5.31 CPU) @ 90
+857.63/s (
n=482454)  
                     Rate       blackadder theorbtwo_assign        the
+orbtwo
blackadder        71109/s               --             -22%           
+  -59%
theorbtwo_assign  90858/s              28%               --           
+  -47%
theorbtwo        171489/s             141%              89%           
+    --

[download]
Abigail

In reply to Re: Unfortunately benchmarking things with $& isn't easy... by Abigail-II
in thread Obtaining server name from UNC path by blackadder

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