For searching purposes, this data structure is sometimes called a trie.

I don't like your eval solution, so here are some alternatives. The first solution is yours, the second used the Data::Diver module, the third is just a plain loop.

use warnings; use strict;
use Data::Dump::Streamer;

my @word = map { lc } qw"
    the in of to that
";

# solution in OP
my %x;
for (my @w = @word) { s/(.)/\{\1\}/g; eval("\$x${_}{EOS}++"); }
Dump(\%x);

# module
use Data::Diver "DiveVal";
my %y;
for (@word) { DiveVal(\%y, split(//, $_), "EOS")++; }
Dump(\%y);

# by hand
my %z;
for (@word) { 
    my $t = \%z;
    for my $k ($_ =~ /./gs) { $t = \%{$$t{$k}} }
    $$t{EOS}++;
}
Dump(\%z);

__END__
[download]

Output:

\1 better written as $1 at a.pl line 10.
$HASH1 = {
           i => { n => { EOS => 1 } },
           o => { f => { EOS => 1 } },
           t => {
                  h => {
                         a => { t => { EOS => 1 } },
                         e => { EOS => 1 }
                       },
                  o => { EOS => 1 }
                }
         };
$HASH1 = {
           i => { n => { EOS => 1 } },
           o => { f => { EOS => 1 } },
           t => {
                  h => {
                         a => { t => { EOS => 1 } },
                         e => { EOS => 1 }
                       },
                  o => { EOS => 1 }
                }
         };
$HASH1 = {
           i => { n => { EOS => 1 } },
           o => { f => { EOS => 1 } },
           t => {
                  h => {
                         a => { t => { EOS => 1 } },
                         e => { EOS => 1 }
                       },
                  o => { EOS => 1 }
                }
         };
[download]

Anyway, thanks for your contributions. I'll make one-liners out of those :)

I'd make two minor adjustments to your one-liner plus one major one and fix one bug:

perl -MStorable -ne 's/(.)/\{$1\}/g;eval­("\$x$_\{­EOS}=0");END{sto­re
+(\%x,out­put)}'
#              (Perl not sed)^       prettier^^^  bug^^   ^^^^was quit
+e wasteful
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Regarding

I'll make one-liners out of those :)

Just because it is a one-liner doesn't mean it has to suck as code (using string-eval like that). ;) The approaches I've used (for example in Re: find all paths of length n in a graph (Boggle solver)) fit easily as one-liners. I've replaced your 'EOS' with a more universal "end of" string, "\n".

perl -MStorable -ne '$p=\%t;$p=$p->{$_}||={}for/./gs;END{sto­re(\%t,ou
+t­put)}'
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That doesn't even autovivify.

Here's another version for those who don't care about the 'store to file' step (with extra verbosity to overcome Data::Dumper's extremely ugly defaults and to provide sample input). Note that here I use an earlier approach of mine so I can have the end-of-word entry point to a false value instead of to an empty hash.

grep '^s\?[hk]\?[ei][ln][tlk]y\?$' /usr/share/dict/words | perl -MData
+::Dumper -ne '$p=\\%t;$p=\$$p->{$_}for/./gs;$$p=0;END{print Data::Dum
+per->new([\%t])->Terse(1)->Indent(1)->Sortkeys(1)->Useqq(1)->Dump()}'
+ | less
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- tye        

I think you didn't understand the purpose of my post. I'm talking about a perl one-liner.
It goes without saying that I wouldn't use such approach in a proper perl program.