my %hash = (-5189 => 63, -3213 => 9, -2357 => 3);

sub next_key {
  my ($h,$key) = @_;

  foreach my $hkey (sort keys %$h) {
   return $h->{$hkey} if $hkey >= $key;
  }
  return;
}
print next_key(\%hash,3000) . "\n";