Recursion will help you there. (Doing it iteratively instead is also possible, of course, but less natural and elegant.) Here's one solution:

#!/usr/bin/perl

use strict;
use warnings;

my %hash = (
    "Usb1"      => "Usb2,Usb3",
    "Usb2"      => "val1,val2,Usb4",
    "Usb3"      => "val3",
    "Usb4"      => "val4"
);

say_values(\%hash, "Usb1");
say_values(\%hash, "Usb2");

sub say_values {
    print_values(@_);
    print "\n";
}

sub print_values {
    my ($hash_ref, $key) = @_;
    return unless exists $hash_ref->{$key};

    my @values = split ",", $hash_ref->{$key};
    foreach my $index (0..$#values) {
        print $values[$index];
        if(exists $hash_ref->{$values[$index]}) {
            print "[";
            print_values($hash_ref, $values[$index]);
            print "]";
        }
        print "," unless $index == $#values;
    }
}
[download]

This produces:

$ perl 1098967.pl
Usb2[val1,val2,Usb4[val4]],Usb3[val3]
val1,val2,Usb4[val4]
$
[download]