Yhis may work better (UNTESTED):

use warnings;
use strict;


open my $IN2, '<', 'infile2.csv' or die "Cannot open 'infile2.csv' bec
+ause: $!";

my %data;
while ( <$IN2> ) {
    my ( $key, $value ) = split /,/;
    $data{ $key } = $value;
    }

close $IN2;


open my $OUTFILE, '>', 'outfile.csv' or die "Cannot open 'outfile.csv'
+ because: $!";
open my $IN1, '<", 'infile1.csv' or die "Cannot open 'infile1.csv' bec
+ause: $!";

while ( <$IN1> ) {
    chomp;
    print $OUTFILE $_;
    if ( exists $data{ $. - 1 ) ) {
        print $OUTFILE ",$data{$. - 1}\n";
        }
    }

close $IN1;
close $OUTFILE;
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