my @monks = ...;  # Array of Monk objects, with a level method.
my %monks_by_level;
push(@{$monks_by_level {$_->level()}}, $_) for @monks;
while (my ($level, $monks) = each(%monks_by_level)) {
    callToPrayer($monks)
}

##</code><code>##

foreach my $level (1 .. 10) {
    callToPrayer(grep($_->level() == $level, @monks));
}