This code is just not going to work... @list = <IN>; #slurps whole into @list Your code: 
while (<IN>)
{
   my @list1 = split ' ', <IN>;
   my @list2 = split ' ', <IN>;
   my @diff  = pairwise { $b - $a } @list2, @list1;
}

[download]
Why would you think that @list1 and @list2 would have alternating lines? Curious minds would like to know what your thinking was in this code? Once you have read <IN>, what makes you think that you can do it again in the list2 assignment? <IN> is now "empty".

User input, like disk file input can only be read once without asking the user or the disk to Please "read it again for me". -----------------------------

If I understood your requirement correctly, this is a straight-forward implementation. 

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my @all_lines;
my @this_record;

### parse the data ###
while (<DATA>)
{    
    #chomp not needed as we are splitting on whitespace
    
    if (/^\d/)
    {
       push @this_record, split;
    }
    else  #this is the blank line "separator", end-of-record
    {
       push @all_lines, [@this_record]; 
       @this_record =();
    }
}
push @all_lines, [@this_record];  # the last record at EOF

### print the result ###

my $ref_current_line = $all_lines[0];

foreach my $ref_next_line (@all_lines[1..@all_lines-1])
{
   print "@$ref_current_line\n";
   print "@$ref_next_line\n";
   print "-----\n";
   
   my @A = @$ref_current_line;
   my @B = @$ref_next_line;
   
   while ( (defined($a =shift @A) and defined( $b=shift @B)) )
   {
      printf ("%6.3f ", $a-$b);
   }
   print "\n";
   
   $ref_current_line = $ref_next_line;
   print "\n"; 
}

=printout
42.034 41.630 40.158 26.823 26.366 25.289 23.949
34.712 35.133 35.185 35.577 28.463 28.412 30.831
-----
 7.322  6.497  4.973 -8.754 -2.097 -3.123 -6.882 

34.712 35.133 35.185 35.577 28.463 28.412 30.831
33.490 33.839 32.059 32.072 33.425 33.349 34.709
-----
 1.222  1.294  3.126  3.505 -4.962 -4.937 -3.878 
=cut


__DATA__
42.034 41.630 40.158 26.823 26.366 
25.289 23.949

34.712 35.133 35.185 
35.577 28.463 28.412 30.831

33.490 33.839 32.059 32.072 33.425 33.349 
34.709

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In reply to Re: subtraction in array by Marshall
in thread subtraction in array by Sidac

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