$ perl -le'
my @array = qw/
    cow
    cowboy
    boy
    cat
    do
    dog
    /;

my $string = join q//, @array;
print $string;
1 while $string =~ s/(\w{2,})\1/$1/;
print $string;
'
cowcowboyboycatdodog
cowboycatdog