my %done;
my @data = (1..100);
for (1..100) {
    push @data, int (rand() * 100);
}
for my $item (@data) {
    next if exists $done{$item};
    #  do something time consuming
    # ...
    $done{$item}++;
}

##</code><code>##

use Benchmark qw {:all};
use 5.016;

my %hash;

#  set up the hash
for (1001..2000) {
    $hash{$_}++;
}
#  two keys we use below
our $key1 = 1001;
our $key2 = 1002;
#  hash key 1 is SV
$hash{$key1} = 1;
#  hash key 2 is RV
$hash{$key2} = {1..10};

#  assign to global as a baseline
our $xx_global;

#  keys are short so the timing table is not too wide
#  char  1:   e  = exists check, 
#             v  = value check
#  chars 2,3: ck = constant key, 
#             vk = variable key
#  chars 4,5: sv = key contains scalar value, 
#             rv = key contains reference
#  char  6:   l  = assign to lexical, 
#             g  = assign to global
#  thus
#  ecksvl means "exists check 
#                using constant key 
#                containing a scalar value, 
#                assigned to lexical"
#  (the value is clearly redundant for an exists check, 
#   but is retained for completeness)

my %checks = (
    ecksvl => 'my $x = exists $hash{1001}',
    evksvl => 'my $x = exists $hash{$key1}',
    vcksvl => 'my $x = $hash{1001}',
    vvksvl => 'my $x = $hash{$key1}',
    evksvg => '$xx_global = exists $hash{$key1}',
    vvksvg => '$xx_global = $hash{$key1}',
    eckrvl => 'my $x = exists $hash{1002}',
    evkrvl => 'my $x = exists $hash{$key2}',
    vckrvl => 'my $x = $hash{1002}',
    vvkrvl => 'my $x = $hash{$key2}',
    evkrvg => '$xx_global = exists $hash{$key2}',
    vvkrvg => '$xx_global = $hash{$key2}',
);


cmpthese (
    -3,
    \%checks
);

##</code><code>##

             Rate evksvl evkrvl ecksvl eckrvl evksvg evkrvg vvksvl vvkrvl vckrvl vcksvl vvksvg vvkrvg
evksvl 10733145/s     --    -5%    -7%   -12%   -15%   -18%   -29%   -31%   -32%   -33%   -41%   -48%
evkrvl 11290643/s     5%     --    -2%    -7%   -10%   -14%   -25%   -27%   -28%   -29%   -38%   -45%
ecksvl 11570664/s     8%     2%     --    -5%    -8%   -12%   -23%   -25%   -27%   -27%   -36%   -44%
eckrvl 12176232/s    13%     8%     5%     --    -3%    -8%   -19%   -21%   -23%   -23%   -33%   -41%
evksvg 12572221/s    17%    11%     9%     3%     --    -5%   -17%   -19%   -20%   -21%   -31%   -39%
evkrvg 13168623/s    23%    17%    14%     8%     5%     --   -13%   -15%   -17%   -17%   -28%   -36%
vvksvl 15082826/s    41%    34%    30%    24%    20%    15%     --    -2%    -4%    -5%   -17%   -27%
vvkrvl 15461840/s    44%    37%    34%    27%    23%    17%     3%     --    -2%    -3%   -15%   -25%
vckrvl 15777625/s    47%    40%    36%    30%    25%    20%     5%     2%     --    -1%   -13%   -23%
vcksvl 15909705/s    48%    41%    38%    31%    27%    21%     5%     3%     1%     --   -13%   -23%
vvksvg 18207860/s    70%    61%    57%    50%    45%    38%    21%    18%    15%    14%     --   -12%
vvkrvg 20580512/s    92%    82%    78%    69%    64%    56%    36%    33%    30%    29%    13%     --