use List::Util qw/ min max sum /;
$a = -55;
$b = -54;
$c = -55;
$d = -54;
for (1..110)# Start your loop
{
$file = "x_$a\_to_$b\_y$c\_to_$d";#file name

open(INFILE,"<","$file") or die("Couldn't open file for reading!\n");

while(<INFILE>){
    push @column8,(split(/\s+/,$_))[7];##Push data to array
}
if(@column8)## if array is not empty then it will go inside
{
$ZMAX = max @column8;
print "maxZ $ZMAX\n";
$ZMIN = min @column8;
print "minZ $ZMIN\n";
$thick = $ZMAX - $ZMIN;
print "$thick\n";
}
## Once the values are printed this will help you to erase the previous values of $ZMAX, $ZMIN, $thick, @column8 for your second file
$ZMAX="";
$ZMIN="";
$thick="";
@column8=();
$c++;
$d++;

## Note this loop will run for 110 times so it seems you should have 110 files when you increment $c++ and $d++; 
}