Quiz 23: Find all cavities within given blocks

Problem:

We have N x N Blocks. Each block have a depth ranging from 0-9 mts. Now a block is called a "Cavity" if it is not in corner row or column and all other neighboring blocks have less depth. Neighbor is any block which share a common side.
Find all such cavities

Input Format: 
N

Followed by N lines with depth of each block

Output Format: 
Replace all cavities by X

Constraints: 
none

Sample Input

7
4111114
1191111
3111103
1111541
1516132
1111111
6666666

Sample Output:
4111114
11X1111
3111103
1111X41
1X1X132
1111111
6666666

Explanations:-

line 2, 3rd element 9 is greater than all neighbors, so it is a cavity and replaced by X

---

Solution:

chomp($n=<STDIN>);
$str1;
for($i=0;$i<$n;$i++)
{
chomp($s=<STDIN>);
$str1=$str1.$s;
}
@arr=split("",$str1);
$len=@arr;
for($i=$n;$i<$len-$n-1;$i++)
{
if($i%$n==0 or $i%$n==$n-1)
{
next;
}
elsif($arr[$i]>$arr[$i+1] and $arr[$i]>$arr[$i-1] and $arr[$i]>$arr[$i-$n] and $arr[$i]>$arr[$i+$n])
{
if($arr[$i-$n] eq "X" or $arr[$i-1] eq "X")
{
}
else
{
$arr[$i] = X;
}
}
}
for($i=1;$i<$len+1;$i++)
{
print "$arr[$i-1]";
if($i%$n == 0)
{
print "\n";
}
}

---

Tips:

Make a single array and then check if element is cavity