for (1..110)
{
......//Only when this loops ends you are assigning zmax
}
$ZMAX = max @column8;
print "maxZ $ZMAX\n";
$ZMIN = min @column8;
print "minZ $ZMIN\n";
$thick = $ZMAX - $ZMIN;
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The OPed code actually assigns  $ZMAX etc. within the body of the for-loop.