If I understand you correctly, you want to compare the difference of each line with 70% of the value of the *first line* third column value. If that is the case, simply do this:

my $minvalue;
foreach (@temp){
  my @t= split("\t",$_);
  $minvalue= $t[2] if (not defined $minvalue);
  if( $x =~ ($t[4]-$t[3]) > 70 * $minvalue/100) {
    ...
[download]

This method works whether you have the whole file in memory or are reading it line by line from a file.

Note that you can move the calculation of the 70% to the line where you initialize $minvalue, so that it is calculated once instead of every time through the loop.