The trick is to work from the inside-out. Define the lowest level, then add that to the next level. Inside the foreach, I can use $hash on both sides of the assignment because Perl will compute the right hand side first then assign that result to the left-hand side.

my @tmp = qw( a b c d );
my @keys = reverse @tmp;

my $first_key = shift @keys;
my $hash = { $first_key => [ 0 ] };

foreach my $key ( @keys )
    {
    $hash = { $key => $hash };
    }
    
use Data::Dumper;
print Dumper( $hash );
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Rael Dornfest asked me to do this for bloxsom, and I thought I had posted it somewhere but I can't find it now.

You can make it simpler sometimes by putting the leaf value as the initial value for $hash, then letting the foreach construct all the levels of the hash. It's a bit cleaner, but harder to see the trick:

my @tmp = qw( a b c d );

my $hash = [0]; # not really a hash

foreach my $key ( reverse @tmp )
    {
    $hash = { $key => $hash };
    }
    
use Data::Dumper;
print Dumper( $hash );
[download]