Hmmm, well now I'm a little surprised. I was going to argue that if you're going to benchmark then you must compare apples to apples. In your example, 'verbose' uses an extra variable 'my $key' which the mapping version does not. I thought that might be a factor, so I made a new function 'verbose2' which uses $_ like the mapping function, expecting that it might be on par (or at least faster than 'verbose').

But using $_ in place of $key was actually slower ???? So I tried one more time with 'verbose3' to re-write the function exactly the same as the mapping version, only using foreach instead. In my mind, verbose2 and verbose3 are exactly the same code and Perl should have interpreted them to be the same at run time, but again verbose3 was slower yet.

Can a Perl innards expert explain why verbose2 is slower than verbose? Or why verbose3 is slower than verbose2?

#!/usr/bin/perl 
use strict;
use warnings;

use Benchmark qw(:all);

my %h;

@h{'A'..'Z','a'..'z'} = 1..52;

sub verbose {
   my $hash = shift;
   foreach my $key (sort keys %$hash) {
      print "$key: $hash->{$key}\n";
   }
}

sub verbose2 {
   my $hash = shift;
   foreach (sort keys %$hash) {
      print "$_: $hash->{$_}\n";
   }
}

sub verbose3 {
   my $hash = shift;
   print "$_: $hash->{$_}\n" foreach sort keys %$hash;
}

sub idiom {
   my $hash = shift;
   print map "$_: $hash->{$_}\n", sort keys %$hash;
}

timethese(1000000, {
   'Verbose' => 'verbose(\%h)',
   'Verbose2' => 'verbose2(\%h)',
   'Verbose3' => 'verbose3(\%h)',
   'Idiom' => 'idiom(\%h)',
});

timethese(10000000, {
   'Verbose' => 'verbose(\%h)',
   'Verbose2' => 'verbose2(\%h)',
   'Verbose3' => 'verbose3(\%h)',
   'Idiom' => 'idiom(\%h)',
});
[download]

Results:
Benchmark: timing 1000000 iterations of Idiom, Verbose, Verbose2, Verb
+ose3...
     Idiom:  1 wallclock secs ( 0.85 usr +  0.00 sys =  0.85 CPU) @ 11
+76470.59/s (n=1000000)
   Verbose:  1 wallclock secs ( 1.26 usr +  0.00 sys =  1.26 CPU) @ 79
+3650.79/s (n=1000000)
  Verbose2:  0 wallclock secs ( 1.34 usr +  0.00 sys =  1.34 CPU) @ 74
+6268.66/s (n=1000000)
  Verbose3:  2 wallclock secs ( 1.39 usr +  0.00 sys =  1.39 CPU) @ 71
+9424.46/s (n=1000000)
Benchmark: timing 10000000 iterations of Idiom, Verbose, Verbose2, Ver
+bose3...
     Idiom:  8 wallclock secs ( 8.57 usr +  0.00 sys =  8.57 CPU) @ 11
+66861.14/s (n=10000000)
   Verbose: 12 wallclock secs (12.92 usr +  0.00 sys = 12.92 CPU) @ 77
+3993.81/s (n=10000000)
  Verbose2: 13 wallclock secs (13.06 usr +  0.00 sys = 13.06 CPU) @ 76
+5696.78/s (n=10000000)
  Verbose3: 15 wallclock secs (13.90 usr +  0.01 sys = 13.91 CPU) @ 71
+8907.26/s (n=10000000)
[download]