Dynamic Loop outputs the same on each iteration - perl

Question

I am attempting to write a script to automate some data collection. Initially the script runs a series of commands which are carried out by the system. The output of these commands is stored in two text files. Following data collection, I am attempting to implement a for loop so that a third output file is generated which lists the value of interest from the first line of output 1 and the second line of output one, as well as the relative error. The following code completes the correct number of times, but returns the same values on all four lines. I suspect this has to do with the filehandler variable, but am unsure how to solve the issue.
for($ln = 1; $ln<5;$ln++){
open($fh, '<',"theoretical.dat",<$ln>)
or die "Could not open file 'theoretical.dat' $!";
#line = split(' ',<$fh>);
$v = $line[3];
open($fh2, '<',"actual.dat",<$ln>)
or die "Could not open file 'actual.dat' $!";
#line = split(' ',<$fh2>);
$v0 = $line[3];
$e = abs(($v0-$v)/$v0);
$rms = $rms + $e^2;
my #result = ($v, $v0, $e);
print "#result \n";
}
The output file code has been omitted. It contains an if/else depending upon if output should be piped into results.dat or appended.
Note that the data in question is stored in as 4 numbers per line, only the fourth of which I wish to access with this script. From the output generated it seems that $ln is changing accordingly after each iteration, but the line being read is not despite the argument within the open command which dictates to read line number $ln.
I have tried undefing $fh and $fh2 after each loop, but it still outputs the same.

Answer 1

You can't specify the line number of a file on the open call. In fact reopening a file will cause it to be read again starting from the top.
Without seeing your data files I can't be sure, but I think you want something like this.
Note that you can use autodie instead of coding an explicit test for an open succeeding. You must also use strict and use warnings a the top of every Perl program, and declare all of your variables using my as close as possible to their first point of use. I have declared $rms outside the loop here so that it can accumulate an aggregate sum of squares instead of being destroyed and recreated each time around the loop.
use strict;
use warnings;
use autodie;
open my $theo_fh, '<', 'theoretical.dat';
open my $act_fh, '<', 'actual.dat';
my $rms;
for my $ln (1 .. 5) {
my $v_theo = (split ' ', <$theo_fh>)[3];
my $v_act = (split ' ', <$act_fh>)[3];
my $e = abs(($v_act - $v_theo) / $v_act);
my $rms = $rms + $e ^ 2;
my #result = ($v_theo, $v_act, $e);
print "#result\n";
}