use strict;
use warnings;

sub bin_search {
    my $array = shift;
    my $find  = shift;
    return unless @$array;
    my ($l,$r)=(0,$#$array);
    while ($l<=$r) {
        my $m=int(($l+$r)/2);
        if ($find<$array->[$m]) {
            $r=$m-1;
        } elsif ($find>$array->[$m]) {
            $l=$m+1;
        } else {
            return $m;
        }
    }
    return undef;
}
my @list=map { $_*2 } (1..5);
for (0..11) {
    my $pos=bin_search(\@list,$_);
    print "Search:$_ Position:",defined $pos ? $pos : "undef","\n";
}