pls use,

my($key,$value1,$value2,$value3) = $line =~ /(\w+),(\d+.\d+),(.\d+\s+\
+d+.\d+.)/g;
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In regular expression you have only three group and assigning into four variables, so $value3 will be undef.

Here its the fixed code

use strict;
use warnings;

my %file1Hash;
my $value4;

open my $file1, "<","file1.csv" or die $!;
open my $file2, "<","file2.csv"or die $!;
open my $outfile_1, ">", "combined.rpt.csv" or die $!;

while( my $line = <$file1>){
    chomp $line;
    my($key,$value1) = (split /,/, $line);
    $file1Hash{$key} = $value1;
}

while(my $line1 = <$file2>){

        chomp $line1;
        my($key1) = (split /,/, $line1);

        if (exists $file1Hash{$key1}) {
                print $line1.",".$file1Hash{$key1}."\n";
                    }
        else {
                #  print $line1."\n";
                    }
}

close $file1;
close $file2;
close $outfile_1;

exit 0;
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