use strict;
use warnings;
use Data::Dumper;

use Data::Diver qw( DiveRef );

my $data = {};

while (<DATA>) {
   chomp;
   my @keys = split /:/;
   my $val = pop(@keys);
   my $ref = DiveRef( $data, map \$_, @keys );
   push @$$ref, $val;
}

print Dumper($data);

__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