As has been mentioned in previous questions of this sort, for is faster and requires less memory than map, because map creates a temporary in-memory list to return, which is then transformed into a hash. This effect is much more pronounced for large arrays than small ones... in fact, let's run some benchmarks: 
use strict;
use warnings;
use Benchmark qw(cmpthese);

for my $size (5,50,500) {
  my @lol  = map { [$_, qw(some random useless data)] } (1..$size);
  print "\n\nComparing 'map' and 'for' hash-assignment from array of $
+size elements\n";
  cmpthese (-2, {
    for => sub { my %hash; $hash{$_->[0]} = $_ for @lol;  },
    map => sub { my %hash; %hash = map{$_->[0] => $_} @lol;  } 
  });
}

## OUTPUTS ##

Comparing 'map' and 'for' hash-assignment from array of 5 elements
Benchmark: running for, map, each for at least 2 CPU seconds...
       for:  3 wallclock secs ( 2.22 usr +  0.00 sys =  2.22 CPU) @ 77
+311.15/s (n=171940)
       map:  1 wallclock secs ( 2.19 usr +  0.00 sys =  2.19 CPU) @ 55
+606.20/s (n=122000)
       Rate  map  for
map 55606/s   -- -28%
for 77311/s  39%   --


Comparing 'map' and 'for' hash-assignment from array of 50 elements
Benchmark: running for, map, each for at least 2 CPU seconds...
       for:  2 wallclock secs ( 2.04 usr +  0.00 sys =  2.04 CPU) @ 99
+92.17/s (n=20414)
       map:  2 wallclock secs ( 2.12 usr +  0.00 sys =  2.12 CPU) @ 56
+57.09/s (n=12010)
      Rate  map  for
map 5657/s   -- -43%
for 9992/s  77%   --


Comparing 'map' and 'for' hash-assignment from array of 500 elements
Benchmark: running for, map, each for at least 2 CPU seconds...
       for:  3 wallclock secs ( 2.01 usr +  0.00 sys =  2.01 CPU) @ 94
+8.83/s (n=1910)
       map:  2 wallclock secs ( 2.07 usr +  0.00 sys =  2.07 CPU) @ 33
+5.75/s (n=696)
     Rate  map  for
map 336/s   -- -65%
for 949/s 183%   --

[download]
For large arrays of 500 elements, using the seemingly innocuous map technique is nearly three times slower. For your typical 50-element array, map is almost twice as slow.

I'm actually surprised the performance penalty was so pronounced.

In reply to (map vs. for) optimizing code - need help by MeowChow
in thread optimizing code - need help by Anonymous Monk

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