my $x = "0123456789abcdef" x 4 . "XXX";
my $chunksize= 16;
for (my $i=$chunksize*int((length($x)-1)/$chunksize);$i;$i-=$chunksize) {
    substr($x,$i,0)=" ";
};
print $x;

##</code><code>##

join " ",$x=~/(.{1,$chunksize})/g