#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my @keys = (1,2,3);
my %hash;
@hash{@keys} = (undef) x @keys;

print Dumper \%hash;
__END__
Prints:
$VAR1 = {
          '1' => undef,
          '3' => undef,
          '2' => undef
        };