my @pkgs = qw[ mainpackage-1.0 ];
my %deps;

while (@pkgs) {
    my $pkg = shift @pkgs;
    next if exists $deps{$pkg};

    my @deps = find_dependencies($pkg);
    $deps{$pkg} = \@deps;
    push @pkgs, @deps;
}

##</code><code>##

sub traverse {
    my ($deps, $root, $depth) = @_;
    print "  " x $depth, "$root\n";
    traverse($deps, $_, $depth + 1)
        for @{ $deps->{$root} };
}

traverse( \%deps, "mainpackage-1.0" );

##</code><code>##

mainpackage-1.0
  dependency1-1.3
    dep1_of_dep1
  dependency2-2.3
    dep1_of_dep2
    dep2_of_dep2
    dep3_of_dep3
  dependency3