my @foo = qw( aaaaaaaa bbbbbbbb cccccccc dddddddd eeeeeeee ffffffff);
sub foo {
  for ( @_ ) {
      $_ =~ s/(.)\1/$1/ || return;
      print $_ . "\n";
      foo( $_ );
  }
}

foo @foo;
print @foo;

##</code><code>##

aaaaaaa
aaaaaa
aaaaa
aaaa
aaa
aa
a
bbbbbbb
bbbbbb
bbbbb
bbbb
bbb
bb
b
ccccccc
cccccc
ccccc
cccc
ccc
cc
c
ddddddd
dddddd
ddddd
dddd
ddd
dd
d
eeeeeee
eeeeee
eeeee
eeee
eee
ee
e
fffffff
ffffff
fffff
ffff
fff
ff
f
abcdef