Here is the code I tested with, and it's output:

#!/usr/bin/perl 
my %files=%{{1=>"a", 2=>"b", 34=>"c", 5=>"d", 6=>"e", 7=>"f", 8=>"g", 
+9=>"h"}};
my $max;
print join(" ",keys %files),"\n";
grep($max=($_>$max)?$_:$max,keys %files);

print "<$max>\n";
[download]

perl /tmp/foo 
6 8 1 34 7 9 2 5
<34>
[download]

My guess is that if you try it with your files, you'll find it either works, or your hash doesn't look the way you expect. (Hint: try to output it with Data::Dumper