It all depends on how many things you are looking up, and how many in the match tree.

use strict; use warnings; 



testit(100000,5); 
testit(100000,50); 

testit(1000000,5); 
testit(1000000,50); 

sub testit { 
  my $inputsize=shift; 
  my $testn=shift; 

  my @list; 
  for my $i(1..$inputsize) { push @list,sprintf('%010d',$i); } 

  my @lookfors; 
  for my $i(1..$testn) { push @lookfors,sprintf('%010d',$i); } 

  my %lookup; 
  for my $lf (@lookfors){ $lookup{$lf}=1; } 

{
  my $type='hash'; 
  my $st=time; 
  my $ct=0; 
  for my $test (@list) { 
    unless ($lookup{$test}) {$ct++;} 
    } 
  my $et=time-$st; 
  printf "%5s inputsize:%10d testn:%5d ct:%10d time:%3d \n",$type,$inp
+utsize,$testn,$ct,$et; 
} 


{
# approx an if/the/elsif/else tree 
  my $type='list'; 
  my $st=time; 
  my $ct=0; 
  tests: 
  for my $test (@list) { 
    for my $lf(@lookfors){ if ($test eq $lf) {next tests; } }
    $ct++; 
    } 
  my $et=time-$st; 
  printf "%5s inputsize:%10d testn:%5d ct:%10d time:%3d \n",$type,$inp
+utsize,$testn,$ct,$et; 
} 

}
[download]

result

 hash inputsize:    100000 testn:    5 ct:     99995 time:  0
 list inputsize:    100000 testn:    5 ct:     99995 time:  0
 hash inputsize:    100000 testn:   50 ct:     99950 time:  0
 list inputsize:    100000 testn:   50 ct:     99950 time:  1
 hash inputsize:   1000000 testn:    5 ct:    999995 time:  0
 list inputsize:   1000000 testn:    5 ct:    999995 time:  2
 hash inputsize:   1000000 testn:   50 ct:    999950 time:  0
 list inputsize:   1000000 testn:   50 ct:    999950 time: 10
[download]

As you an see for a small number of items to test against it doesnt make much of a difference. I would use the hash just because the code is cleaner.