my @keys = qw (1,2,3);
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should probably either be:

my @keys = (1,2,3); # or (1..3) if they're consecutive numbers
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or:

my @keys = qw(1 2 3);
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But to get back to your question:

use Data::Dumper;
my @keys = (1,2,3);
my $href = 'somedata';
$href = {pop(@keys) => $href} while @keys;
#$href = {$_ => $href} foreach reverse @keys; # alternate solution, if
+ @keys is constant or needed later
print Dumper( $href );
__END__
$VAR1 = {
          '1' => {
                   '2' => {
                            '3' => 'somedata'
                          }
                 }
        };
[download]

Liz

Update:
Added (1..3), suggested by broquaint

Added foreach reverse alternative, suggested by myself ;-)