Shell Programming and Scripting

h                          # The "h" command copies the pattern buffer into the hold buffer. The pattern buffer is unchanged. 
s/.* //                   # get the content after last space.
s/.\{8\}/& /g          # add space for every 8 chars.
x                          # The "x" command exchanges the hold buffer and the pattern buffer. 
s/[^ ]*$//             # get the content before last space.
G                         # Instead of exchanging the hold space with the pattern space, you can copy the hold space to the pattern space with the "g" command. This deletes the pattern space. If you want to append to the pattern space, use the "G" command. This adds a new line to the pattern space, and copies the hold space after the new line.
s/\n//                   # remove the new line which added by G.

my $str='MD5(secret.txt)= fe66cbf9d929934b09cc7e8be890522e';
$str=~s/(?=(\S{8})+$)/ /g;
print $str;