use strict;
use warnings;

my @strings = qw(
    /help/one/one/one/bar/something_here
    /buy/cash/buy/water/water/water/baz/nothing_here     
);

for (@strings) {
    if (m{(\w+/){2,}}) {
        print "Dup is '$1'\n";
    }
}

=prints
bar/
baz/
=cut
[download]

Even if it did work, you'd still need to add boundary conditions so that a sub directory that is a suffix of the previous directory wouldn't match. Also, if the duplicate is on the end, it wouldn't have a trailing /.

Don't worry, at one point, I also thought a quantifier should work in that way, but that's specifically why they allow \1 in the LHS.