my @a =();
my @b =();

while(<IN>){
  chomp;
  $cnt++;
  /\d+\t(\d+)/;
  if (@a<7){
    push(@a,$1);
  }
  push(@b,$1) if $1 > 3;
  if (@b>7){
    shift(@a);
    shift(@b);
  }
  print $a[0] - $b[-1] . "\n"
}

##</code><code>##

while(<IN>){
  $cnt++
}

##</code><code>##


file size: 70GB
file format :  tsv

1     34
2     6
3     78

simple while-loop time:  

real	10m51.378s
user	9m22.610s
sys	0m13.120s



while loop with regex + push and shift + one subtraction(i forgot to mention that)

real	157m13.245s
user	152m8.520s
sys	1m26.660s