use strict;
use warnings;

my $file2 =<<EOF2;
Code: description
1: Generic user error
2: Error with file location
3: File not found
4: Syntax error
EOF2

my @desc = split m{\n+}, $file2;
my %desc;

for my $line (@desc[1 .. $#desc])   # Skip headings
{
    chomp $line;
    my ($code, $desc) = $line =~ m{ ^ (\d+) : \s* (.+) $ }x;
    $desc{$code} = $desc;
}

<DATA>;     # Skip headings

while (my $line = <DATA>)
{
    chomp $line;
    my @fields = split m{,\s*}, $line;

    if (exists $desc{$fields[1]})
    {
        print join(', ', @fields, $desc{$fields[1]}), "\n";
    }
}

__DATA__
Testname, code, date, file
Test1, 4, 4/11/15, /tmp
Test2, 2, 4/11/15, /log
Test3, 1, 4/11/15, /log
Test4, 7, 4/11/15, /dat
Test5, 4, 4/11/15, /log