Since there's a finite, sequential number of levels, you might as well just use nested arrays:

my (@monks);
while (<DATA>) {
    chomp; split(/ /);
    push(@{$monks[$_[1]]}, $_[0]);
}
mysub($_) for @monks;
sub mysub { print join(' ', sort(@$_)) . "\n"; }

__DATA__
monk1 2
monk2 5
monk3 8
monk4 4
monk5 1
monk6 5
monk7 3
monk8 7
monk9 7
[download]

Or array / hash, if you need to be able to do more than just run through each set of monks:

my (@monks);
while (<DATA>) {
    chomp; split(/ /);
    $monks[$_[1]]{$_[0]} = ();
}
mysub($_) for @monks;
sub mysub { print join(' ', sort(keys %$_)) . "\n"; }

__DATA__
monk1 2
monk2 5
monk3 8
monk4 4
monk5 1
monk6 5
monk7 3
monk8 7
monk9 7
[download]

A hash would enable you to quickly check for specific monks inside each set.