If you use the lookup a lot then it's worth making a lookup table. Consider

use strict;
use warnings;

my %upload = (1 => 'one', 2 => 'two', 3 => 'three', 5 => 'five', 9 => 
+'nine', 10 => 'ten');
my @keys = sort {$a <=> $b} keys %upload;
my %lookup = map {
    $keys[$_] => {
        prev => $_           ? $keys[$_ - 1] : undef,
        next => $_ != $#keys ? $keys[$_ + 1] : undef,
        }
    } 0 .. $#keys;

for my $key (1, 5, 10) {
    if (defined $lookup{$key}{next}) {
        print "$lookup{$key}{next} follows $key\n";
    } else {
        print "$key is the last item\n";
    }

    if (defined $lookup{$key}{prev}) {
        print "$lookup{$key}{prev} precedes $key\n";
    } else {
        print "$key is the first item\n";
    }
}
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Prints:

2 follows 1
1 is the first item
9 follows 5
3 precedes 5
10 is the last item
9 precedes 10
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my %lookup = map {$keys[$_] => {prev => $keys[$_ - 1], next => $keys[$
+_ + 1]}} 0 .. $#keys;
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Update: see johngg's reply below!