@hash{@array} = ();

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Benchmark;

sub is_elem_of_A {
        my ($x,  %hash) = shift;
        @hash{@_} = (1) x @_;
        exists $hash{$x}
}

sub is_elem_of_B {
        my ($x,  %hash) = shift;
        @hash{@_} = ();
        exists $hash{$x}
}

my %param = (
        short => [ qw(one one two three four five) ],
        long => [ 10, 1 .. 10000 ],
);

for(qw(short long)) {
        print "Timing with $_ list\n";
        my $list = $param{$_};
        timethese -5, {
                xop => sub{ is_elem_of_A @$list },
                emptylist => sub{ is_elem_of_B @$list },
        };
}
__END__

Timing with short list
Benchmark: running emptylist, xop, each for at least 5 CPU seconds...
 emptylist:  6 wallclock secs ( 5.29 usr +  0.04 sys =  5.33 CPU) @ 42362.48/s (n=225792)
       xop:  6 wallclock secs ( 5.35 usr +  0.04 sys =  5.39 CPU) @ 31733.58/s (n=171044)
Timing with long list
Benchmark: running emptylist, xop, each for at least 5 CPU seconds...
 emptylist:  5 wallclock secs ( 5.28 usr +  0.01 sys =  5.29 CPU) @ 22.12/s (n=117)
       xop:  6 wallclock secs ( 5.50 usr +  0.02 sys =  5.52 CPU) @ 15.40/s (n=85)