Check to see if the key is in there, otherwise, subtract one at a time until it's found. Cache this, too.

my $nearest_key = nearest(\%hash, $key);

{
  my %nearest;
  sub nearest {
    my ($href, $k) = @_;
    my $low = $k;

    return $nearest{$k} if exists $nearest{$k};

    $low-- until exists $href->{$low};

    @nearest{$low .. $k} = ($low) x ($k - $low + 1);
    return $low;
  }
}
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japhy -- Perl and Regex Hacker