>perl -wMstrict -le
"$ARGV[$#ARGV] =~ m{ (\S) \s* \z }xms;
 my ($lastchr, $lastpos) =
   defined $1 ? ($1, $-[1]) : ('absent', -1);
 print '----- output -----';
 print qq{last non-ws char is '$lastchr' at index $lastpos};
"
"Message Line 1" "Message Line 2" "Message Line 3"
"Prompt:             "
----- output -----
last non-ws char is ':' at index 6