how can the script know?

The script doesn't know anything, ever. It's you who knows, or doesn't, and the script reflects that knowledge.

If I have an array with 1,2,5,7,10, how do I determine what index 5 is? I know it's 2

You make another hash, keyed with the contents of your array slots, whose values are their indices:

# (merging previous post)
my @sorted_indices = sort {$a <=> $b} keys %upload;
my %lookup_hash;
@lookup_hash{@sorted_indices} = 0 .. $#sorted_indices;
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Now you lookup e.g. 5 in %lookup_hash:

my $i = $lookup_hash{5};
print $x,"\n"; # prints 2

# get the next element from the hash
my $next = $sorted_indices[$x + 1]; # $next holds 7

# get your image
my $image = $uploads{$next};
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_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}