This is fine if you have just a few keys. However, this approach is O(n) (i.e. execution time increases linearly with the number of items in the set), whereas a hash has approximately constant lookup time — or O(1).

Even for only 10 items, a hash is considerably faster:

#!/usr/bin/perl -w

my $nkeys = 10;

my @keys;
our $key; # (keep last key for lookup below)
for (1..$nkeys) {
    # generate random keys consisting of 3..10 chars
    $key = join "", map chr(97+int(rand(26))), (1..3+int(rand(8)));
    push @keys, $key;
}

# setup 'if'-code
my $code = "sub isMember_if {\n"
           . join("", map "return 1 if \$_[0] eq '$_';\n", @keys)
           . "return 0;}";
#print "$code\n";
eval $code;

# setup hash
my %hash = map { $_ => 1 } @keys;
sub isMember_hash { return $hash{$_[0]} };


use Benchmark 'cmpthese';

cmpthese( -1, {
    'if'   => 'isMember_if($key)',
    'hash' => 'isMember_hash($key)'
});
[download]

On my machine, this prints (results vary slightly, depending on random keys being used)

          Rate   if hash
if    628278/s   -- -68%
hash 1946613/s 210%   --
[download]

Update: Actually, Perl's hashes are blazingly fast: even for just one key/comparison ($nkeys = 1), the if-approach still doesn't outperform the hash; and with key lengths of more than about 3 chars, the hash lookup is even noticeably faster (which seems to indicate that comparing two identical strings is slower than computing the hashed value of that string...).