use warnings;
use strict;
 
 
open my $in2, '<', 'infile2.csv' or die "Cannot open 'infile2.csv' because: $!";
 
my %data;
while ( <$in2> ) {
    my ( $key, $value ) = split /,/;
    $data{ $key } = $value;
    }
 
close $in2;
 
 
open my $outfile, '>', 'outfile.csv' or die "Cannot open 'StatsCubeR2format.csv' because: $!";
open my $in1, '<' , 'infile1.csv' or die "Cannot open 'infile1.csv' because: $!";
 
while ( <$in1> ) {
    chomp;
    print $outfile "$_";
    if ( exists $data{ $. - 1 } ) {
        print $outfile ",$data{$. - 1}\n";
        }
    else {
                print $outfile ",field_name\n";
                }
    }
 
close $in1;
close $outfile;