Or a lookup table?

my %table = ( 561231 => 'whatever',
              700000 => 'whatevr2',
              ... );

my @thresh = sort { $a <=> $b } keys %table;
$_ = pop(@thresh) while $thresh[0] < $numbers;
my $contig = $table{$_};
[download]

Even this can be improved if there's more than ~200 thresholds by using a binary search method instead. I wouldn't bother with that unless it proves to be a bottleneck, though.