use strict;

my @List;

open(FILE, "filename.txt") or die "Unable to open file\n";

# assuming numbers, get them from file and add them to your array

while(<FILE>){
   #each time through the loop, line is held in $_
   chomp $_; #remove any newline from the line taken from file
   push(@List,$_); #push line into the array, in your case the number you got
}   

close(FILE);

# to loop through the array, I found it usefull to increment my count var ($i) by two
# here, scalar(@List), returns the number of elements in the array.

for(my $i=0;$i < scalar(@List);$i+=2){

   my $answer = ($List[$i+1]-$List[$i]); #use array indexing for your calculation
   print "$answer\n";

}