Not to mention the fact that the original code has a bug. There needs to be parentheses around the "1" to force list context.

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

my (%hash1, %hash2);
my @keys = qw/foo bar baz/;
@hash1{@keys} =  1  x 3;
@hash2{@keys} = (1) x 3;
print Dumper \%hash1, \%hash2;
[download]

That generates:

$VAR1 = {
          'foo' => '111',
          'baz' => undef,
          'bar' => undef
        };
$VAR2 = {
          'foo' => 1,
          'baz' => 1,
          'bar' => 1
        };