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__

##</code><code>##

\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 }
                }
         };