s/^(.{6,}?)\s+\z/$1/s;
s/^\s*(.{6}).*\z/$1/s;

##</code><code>##

s/(?<=.{6})\s+\z//s;

##</code><code>##

s/.{6}\K\s+\z//s;     # Req 5.10

##</code><code>##

s/^\s+(?=.{6})//s;
s/(?<=^.{6}).*\z//s;

##</code><code>##

s/^\s+(?=.{6})//s;
s/(?<=.{6}).*\z//s;

##</code><code>##

s/^\s+(?=.{6})//s;
s/^.{6}\K.*\z//s;     # Req 5.10