I agree the  unpack solution is better, but a regex solution without the  split complication or  push-ing would be something like:

>perl -wMstrict -le
"my $s = 'GRID     213736         15235.2 -2606.28596.178';
 my @ra = $s =~ m{ .{1,8} }xmsg;
 print qq{:$_:} for @ra;
"
:GRID    :
: 213736 :
:        :
:15235.2 :
:-2606.28:
:596.178:
[download]

Update: Did not notice balakrishnan's earlier and essentially identical reply before posting this.