Vertical index Perl - perl

Question

File 1 has ranges 3-9, 2-6 etc
3 9
2 6
12 20
File2 has values: column 1 indicates the range and column 2 has values.
1 4
2 4
3 5
4 4
5 4
6 1
7 1
8 1
9 4
I would like to calculate the sum of values (file2, column2) for ranges in file1). Eg: If range is 3-9, then sum of values will be 5+4+4+1+1+1+4 = 20
What I have tried is:
open (FILE1,"file1.txt");
open (FILE2,"file2.txt");
#file1 = <FILE1>;
#file2 = <FILE2>;
foreach (#file1)
{
#split_file2 = split("\\s",$_); //splitting the file by space
foreach (#file2)
{
#split_file2 = split("\\s",$_); //splitting the file by space
if (#split_file1[0] == #split_file2[0]) //if column0 of file1 matches with column0 of file2
{
$x += #split_file2[1]; //sum the column1 of file2
if ( #split_file2[0] == #split_file1[0] ) //until column1 of file1 = column0 of file2.
{
last;
}
}
}}

Answer 1

Always use use strict; use warnings;.
split /\s/ is easier to read. split ' ' is what you actually want.
Don't use global variables (e.g. for file handles).
It's useful to check if open succeeds, if only by adding or die $!.
Use meaningful names, not file1 and file2.
use strict;
use warnings;
use feature qw( say );
use List::Util qw( sum );
my $file1 = 'file1.txt';
my $file2 = 'file2.txt';
my #file2;
{
open(my $fh, '<', $file2)
or die "Can't open $file2: $!\n";
while (<$fh>) {
my ($k, $v) = split;
$file2[$k] = $v;
}
}
{
open(my $fh, '<', $file1)
or die "Can't open $file1: $!\n";
while (<$fh>) {
my ($start, $end) = split;
say sum grep defined, #file2[$start .. $end];
}
}

Answer 2

Another solution:
#!/usr/bin/perl
use strict;
use warnings;
my $f1 = shift;
my $f2 = shift;
open FH1, "<", $f1 or die "$!\n";
open FH2, "<", $f2 or die "$!\n";
my %data;
while (<FH1>) {
$data{$1} = $2 if ($_ =~ m/^(\d+)\s+(\d+)$/);
}
while (<FH2>) {
if ($_ =~ m/^(\d+)\s+(\d+)$/) {
my $sum;
for ($1..$2) {
$sum += $data{$_} if defined($data{$_});
}
print "sum for $1-$2: $sum\n" if defined($sum);
}
}
close FH1;
close FH2;
Call: script.pl values.txt ranges.txt