use strict; 
use warnings; 
 
my $new_data; 
my $counter = 1; 
my $class_watch; 
my $inbetween = 1; 
 
my $line_before; 
 
while (<DATA>){ 
    chomp; 
    my ($chrom_pos, $all1, $all2, $class) = split /\s/; 
     
    #if the class changes save the $chrom_pos and the line before 
    if($class ne $class_watch ){ 
         
        $new_data .= "$chrom_pos\n" if $inbetween <= 2; 
        $new_data .= "$line_before \n" if $inbetween > 1; 
         
        #reset counter data 
        $inbetween = 1; 
        $class_watch = $class; 
        $line_before = $chrom_pos; 
    }else { 
        #if the class is the same keep track 
        $inbetween++; 
        $line_before = $chrom_pos; 
        $class_watch = $class; 
    } 
    #line counter 
    $counter++; 
} 
 
print $new_data; 
 
__DATA__ 
1457 G G SAME  
1979 G G SAME  
2056 T T SAME  
3091 A A SAME  
3562 A G DIFF  
3778 A A SAME  
4124 T T SAME  
4229 C T DIFF  
4571 A G DIFF  
5019 A C DIFF  
5114 C C SAME  
6291 T T SAME  
6414 C C SAME  
6553 C C SAME  
6941 G G SAME 
EOF