use strict;
use warnings;

my $str = "One:Two:Three:four, One:Two:3:4, One:2:3:4, One:2:three";

my %hash;

my @defs = split(/\s*,\s*/, $str);  # Use Text::CSV?
foreach my $def (@defs) {
   my @keys = split(/:/, $def);
   my $val = pop(@keys);
   my $p = \\%hash;
   while (@keys) { $p = \($$p->{shift(@keys)}); }
   $$p = $val;
}

require Data::Dumper;
print(Data::Dumper::Dumper(\%hash));

##</code><code>##

use strict;
use warnings;

use Data::Diver qw( DiveVal );

my $str = "One:Two:Three:four, One:Two:3:4, One:2:3:4, One:2:three";

my %hash;

my @defs = split(/\s*,\s*/, $str);  # Use Text::CSV?
foreach my $def (@defs) {
   my @keys = split(/:/, $def);
   my $val = pop(@keys);
   DiveVal(\%hash, map { \$_ } @keys) = $val;
}

require Data::Dumper;
print(Data::Dumper::Dumper(\%hash));