It doesn't matter. Your loop administration isn't the bottleneck, the method call dominates the running time.

But how many parameters do you have anyway? If you look at the total program, is the loop going to be the bottleneck? Optimizing something that only takes a small part of the running time isn't very useful.

Here are some benchmarking results, which show that it's the method call being the bottleneck.

Abigail

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

use Benchmark::Sized;  # To be released...
use CGI;

use vars qw /$cgi/;

$cgi = CGI -> new;
   
my $stats = sized_timethese
    run_for     =>  1,
    start_size  =>  2,
    end_size    =>  4096,
    steps       =>  12,
    seed        =>  sub {
        $main::cgi -> delete_all;
        foreach (1 .. $_ [0]) {$main::cgi -> param ($_, $_)}
    },
    code        =>  {
        map1    =>
           'my %input = ();
            map  {$input {$_} = $main::cgi -> param ($_)} $main::cgi -
+> param',
        map2    =>
           'my %input =
            map {$_ => $main::cgi -> param ($_)} $main::cgi -> param',
        grep    =>
           'my %input = ();
            grep {$input {$_} = $main::cgi -> param ($_)} $main::cgi -
+> param',
        for     =>
           'my %input = ();
            $input {$_} = $main::cgi -> param ($_) for $main::cgi -> p
+aram'
    }
;

print_timed $stats, type => 'sized';

__END__
Runs/second:
Size       for    grep    map1    map2
   2:  9569.16 9774.55 9773.64 9569.16
   4:  5749.09 5749.09 5749.09 5688.57
   8:  3200.00 3169.81 3169.81 3140.19
  16:  1689.62 1659.26 1644.04 1644.04
  32:   872.73  837.38  845.28  853.33
  64:   439.45  426.67  425.71  430.77
 128:   222.22  216.22  214.29  216.50
 256:   110.19  106.67  106.73  105.71
 512:    53.40   51.38   51.38   50.93
1024:    25.69   24.53   24.53   24.07
2048:    12.50   11.93   11.82   11.54
4096:     6.31    5.94    5.83    5.61
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