sub overlap{ 
    ($_[1] < $_[3] ? $_[1]: $_[3]) 
    - 
    ($_[0] > $_[2] ? $_[0] : $_[2]) + 1; 
}

@data = qw[1 5 2 6]; 
print "\n@data : ", overlap @data 
    and @data = map($_-1, @data) 
        while grep $_>0,@data;
1 5 2 6 :  4
0 4 1 5 :  4
-1 3 0 4 :  4
-2 2 -1 3 :  4
-3 1 -2 2 :  4
-4 0 -3 1 :  4

my @data = qw[1 2 5 6]; 
print "\n@data : ", overlap @data 
    and @data = map($_-1, @data) 
        while grep $_>0,@data;
1 2 5 6 :  -2
0 1 4 5 :  -2
-1 0 3 4 :  -2
-2 -1 2 3 :  -2
-3 -2 1 2 :  -2
-4 -3 0 1 :  -2

my @data = qw[2 5 1 6]; 
print "\n@data : ", overlap @data 
    and @data = map($_-1, @data) 
        while grep $_>0,@data;
2 5 1 6 :  4
1 4 0 5 :  4
0 3 -1 4 :  4
-1 2 -2 3 :  4
-2 1 -3 2 :  4
-3 0 -4 1 :  4

my @data = qw[1 6 2 5]; 
print "\n@data : ", overlap @data 
    and @data = map($_-1, @data) 
        while grep $_>0,@data;
1 6 2 5 :  4
0 5 1 4 :  4
-1 4 0 3 :  4
-2 3 -1 2 :  4
-3 2 -2 1 :  4
-4 1 -3 0 :  4