#! 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