#/bin/perl/

use strict;
use warnings;

my $pdb;

my $line;
my @columns;

open $pdb, '<', "structure.pdb" or die $!;

#match string and select column
my $string = "ATOM"; # match for ATOM
my @nth_column;
my $n = 1; # select 2nd column

while ($line = <$pdb>) {
  if ($line =~ $string) {
  chomp $line;
  @columns = split /\s+/, $line;
  push @nth_column, $columns[$n];
  #for testing:
  print "$columns[$n] \n";
  }
}