Well, here's my attempt, assuming that $current holds the value of the current key:

my ($previous,$next);
my @keys = sort { $a <=> $b } keys %hash;

for (my $i=0; $i < scalar @keys; $i+=1) {
    $previous = $keys[$i-1] unless $i == 0;
    $next     = $keys[$i+1] unless $i == scalar @keys;
    last if $keys[$i] == $current;
}
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Alan "Hot Pastrami" Bellows

Update: Oh, this also assumes that the value of $current is known to be in %hash, if a value is in $current that is not in %hash it won't work too good.