Something like this?

#! perl -sw
use 5.010;
use strict;
use List::Util qw[ reduce ];
$a = $b; ## disable Name "main::b" used only once: possible typo
use Data::Dump qw[ pp ];

my %hash = map{
    %{ +reduce{
        +{ $b => ref( $a ) ? $a : [ split ' ', $a ] }
    } reverse split ':', $_ }
} <DATA>;

pp \%hash;

__DATA__
gkey01:key1:1 2 3 4 5 6 7 8
gkey02:key1:key2:key3:1 2 3 4
gkey03:key1:key2:key3:key4:1 2 3 4 5 6 7 8
gkey04:key1:key2:key3:1 2
gkey05:key1:key2:key3:key4:1 2
gkey06:key1:key2:key3:key4:key5:1 2 3
gkey07:key1:key2:1 2 3 4 5 6 7 8 9
gkey08:key1:key2:key3:key4:1 2 3 4 5 6 7
gkey09:key1:1 2 3 4 5 6 7
gkey10:key1:1 2 3 4 5 6
gkey11:key1:1 2 3 4
gkey12:key1:key2:1 2 3 4 5 6 7 8 9 10
gkey13:key1:key2:key3:key4:1 2 3 4
gkey14:key1:key2:key3:1 2
gkey15:key1:1 2 3 4 5 6 7
gkey16:key1:key2:1
gkey17:key1:key2:key3:1 2 3 4 5 6 7 8 9 10
gkey18:key1:key2:1 2 3 4 5 6 7
gkey19:key1:key2:key3:1 2 3 4 5 6 7 8
gkey20:key1:key2:1 2 3 4 5 6 7 8 9
[download]

Output:

C:\test>junk3
{
  gkey01 => { key1 => [1, 2, 3, 4, 5, 6, 7, 8] },
  gkey02 => { key1 => { key2 => { key3 => [1, 2, 3, 4] } } },
  gkey03 => {
     key1 => { key2 => { key3 => { key4 => [1, 2, 3, 4, 5, 6, 7, 8] } 
+} } },
  gkey04 => { key1 => { key2 => { key3 => [1, 2] } } },
  gkey05 => { key1 => { key2 => { key3 => { key4 => [1, 2] } } } },
  gkey06 => {
     key1 => { key2 => { key3 => { key4 => { key5 => [1, 2, 3] } } } }
+ },
  gkey07 => { key1 => { key2 => [1, 2, 3, 4, 5, 6, 7, 8, 9] } },
  gkey08 => {
     key1 => { key2 => { key3 => { key4 => [1, 2, 3, 4, 5, 6, 7] } } }
+ },
  gkey09 => { key1 => [1, 2, 3, 4, 5, 6, 7] },
  gkey10 => { key1 => [1, 2, 3, 4, 5, 6] },
  gkey11 => { key1 => [1, 2, 3, 4] },
  gkey12 => { key1 => { key2 => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] } },
  gkey13 => { key1 => { key2 => { key3 => { key4 => [1, 2, 3, 4] } } }
+ },
  gkey14 => { key1 => { key2 => { key3 => [1, 2] } } },
  gkey15 => { key1 => [1, 2, 3, 4, 5, 6, 7] },
  gkey16 => { key1 => { key2 => [1] } },
  gkey17 => { key1 => { key2 => { key3 => [1, 2, 3, 4, 5, 6, 7, 8, 9, 
+10] } } },
  gkey18 => { key1 => { key2 => [1, 2, 3, 4, 5, 6, 7] } },
  gkey19 => { key1 => { key2 => { key3 => [1, 2, 3, 4, 5, 6, 7, 8] } }
+ },
  gkey20 => { key1 => { key2 => [1, 2, 3, 4, 5, 6, 7, 8, 9] } },
}
[download]

Exactly!! THANKS FOR THE HELP!!!

Go re-read the answers you got to Need some help building a data structure.. You haven't told us enough about the problem you are trying to solve.

Based on the information you've given so far, there is no easy way to build what you seem to be asking for because, for example, ...{key4} can't hold both a hash reference and an array reference. How do you want to handle that?

But really, tell us what the bigger problem is you are trying to solve. I suspect you've latched on to trying to solving an implementation detail where you should be revising an issue with the larger problem.