Thanks for all the great answers, especially those with code samples and benchmarks! It looks like the syntax and space efficiency winner is Set::Light, though not by much (it uses a C module to point all the set members to a single object).
Remember I'm in mod_perl, so construction/deconstruction time is less important than memory footprint and lookup time.
#! /usr/bin/perl -w
use strict;
use Devel::Size qw(total_size);
use Set::Light;
my $lookup = "shave";
my %hash1 = (
shave => '',
the => '',
very => '',
modern => '',
way => '',
);
my %hash2 = (
shave => 1,
the => 1,
very => 1,
modern => 1,
way => 1,
);
my %hash3 = (
shave => undef,
the => undef,
very => undef,
modern => undef,
way => undef,
);
print "Hash3 " . ((exists $hash3{$lookup})?"contains":"does not contai
+n") . " $lookup\n";
my $foo = undef;
my %hash4 = (
shave => \$foo,
the => \$foo,
very => \$foo,
modern => \$foo,
way => \$foo,
);
my $set = Set::Light->new( qw/shave the very modern way/ );
print "Set " . (($set->has($lookup))?"contains":"does not contain") .
+" $lookup\n";
print "Size 1: " . total_size(\%hash1) . "\n";
print "Size 2: " . total_size(\%hash2) . "\n";
print "Size 3: " . total_size(\%hash3) . "\n";
print "Size 4: " . total_size(\%hash4) . "\n";
print "Size S: " . total_size(\$set) . "\n";
__END__
Hash3 contains shave
Set contains shave
Size 1: 363
Size 2: 303
Size 3: 283
Size 4: 315
Size S: 251