my @array = qw( your large, sorted array here );
my $search = "thing";
my $index = bsearch(\@array, $search);

sub bsearch {
    my ($array, $word) = @_;
    my $low = 0;
    my $high = @$array - 1;

    while ( $low <= $high ) {
        my $try = int( ($low+$high) / 2 );
        $low  = $try+1, next if $array->[$try] lt $word;
        $high = $try-1, next if $array->[$try] gt $word;
        return $try;
    }
    return;
}