Use a CSV parsing module, like Text::CSV_XS and read both files into your perl script. To all the twiddling in your script and write your result back. Likely most of that can be done in about three lines of code using csv (). Show us what you tried so far?

If your csv is simple enough, (your fields doesn't have any "," inside), you can match lines with regex. (The properly way to go is using some csv module from cpan (like Tux told))

See perlre and search (?<NAME>pattern) to see the use of %+ at the next example:

#!/usr/bin/perl -w
use strict;

open my $csv1, "<csv1.csv";
open my $csv2, "<csv2.csv";
open my $csv3, ">csv3.csv";

my %csv1_data;
while (<$csv1>)
{
    if (/^(?<first>([^,]*,){3})(?<fields>([^,]*,){3}).*$/)
    {
        $csv1_data{$+{first}}=$+{fields};
    }
}

while (<$csv2>)
{
    if (/^(?<first>([^,]*,){3})(?<fields>([^,]*,){3})(?<rest>.*)$/)
    {
        if (defined $csv1_data{$+{first}})
        {
            print $csv3 "$+{first}$csv1_data{$+{first}}$+{rest}\n";
        }
        else
        {
            print $csv3 $_;
        }
    }
    
}
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