open(FH,"C:\Users\Payal\Desktop\Project bogola\loop.pdb"); print"Enter the first residue\n"; $b=<>; print"Enter the second residue\n"; $c=<>; $d =chomp($b); $e =chomp($c); while($a=) { if($a=~/^ATOM/) { $residue=substr($d..$e,23,1..3); if($residue >=$d and $residue <=$e) { print FH1 $a; open(FH1,">>ichain.pdb") } }}