One way to do this is to transform the hash structure into an AoA so that it is easier to sort, then take first (index 0) and last (index -1) to get min and max. This type of approach allows easy reporting "ties" for min or max if that is important to you, eg maybe the minimum occurs on three different dates. Perl is very good at sorting and this runs faster than one might imagine.

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %years =( 2007 => 
               { '01' => 07,
                 '02' => 34,
                 '10' => 24,
                 '09' => 14,
               },
            2008 =>
               { '01' => 11,
                 '02' => 64,
                 '03' => 20,
                 '09' => 13,
               },           
);

my @year_data;

foreach my $year (keys (%years))
{
    foreach my $month (keys %{$years{$year}})
    {
        push (@year_data, [$year, $month, $years{$year}{$month}]);
    }
}

@year_data = sort { my ($yearA, $monthA, $dataA) = @$a; 
                    my ($yearB, $monthB, $dataB) = @$b;
                               $dataA <=> $dataB
                                      or
                              $monthA <=> $monthB
                                      or
                               $yearA <=> $yearB
                  }@year_data;
                  
print "minimum is: @{$year_data[0]}\n";
print "maximum is: @{$year_data[-1]}\n";

#minimum is: 2007 01 7
#maximum is: 2008 02 64
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