use warnings;
use strict;

use List::Util qw/ min max sum /;

my $a = -55;
my $b = -54;
my $c = -55;
my $d = -54;
for (1..110) {

    my $file = "x_$a\_to_$b\_y$c\_to_$d";

    open(my INFILE, '<', $file) or die "opening '$file': $!";

    my @column8;  # FRESH, NEW, EMPTY ARRAY for each file!
        ## YES defining here again my @column8 is very good
    while(<INFILE>){
        push @column8, (split(/\s+/, $_))[7];
        }
    $ZMAX = max @column8;
    $ZMIN = min @column8;
    $thick = $ZMAX - $ZMIN;
    print "$thick\n";
    $c++;
    $d++;
    }