my @strs = qw/fooxx baryyyy bazzzzzzz/;
s{ ( (.) \2+ ) \z }(substr($1, 0, length($1) / 2))ex, print
  for @strs;

__output__

foox
baryy
bazaaa

##</code><code>##

reverse =~ /\A((.)\2+)/ and substr($_, -(length($1) / 2)) = ''
  for @strs;
print map "$_\n", @strs;

__output__

foox
baryy
bazaaa