Another solution, taking back the idea of a simple key=value file, using a hash, but directly reading from and writing to file, without using File::Tie neither another existing module.

# SimplestConfig.pm

# SimplestConfig->new('filename') returns an object whose attributes
# (simple values only at this time) will be stored in a file named
# filename under the form attribute=value and as such may be usefull
# to manage simple config files.

use strict;
use warnings;

package SimplestConfig;

sub new {
    my ($class, $filename) = @_;
    my $self = {_FileName_ => $filename};
    bless $self, $class;
    if (open CFG, "<$filename") {
        while (<CFG>) {
            /^\s*(\S.*?)\s*=\s*(.*?)\s*$/ and $self->{$1} = $2;
        }
        close CFG;
    } else {
        warn "Can't read $filename, creating it\n";
    }
    return $self;
}

sub DESTROY {
    my ($self) = @_;
    open CFG, ">$self->{_FileName_}" or
        die "Can't write $self->{_FileName_}\n";
    delete $self->{_FileName_};
    print CFG map "$_=$self->{$_}\n", sort keys(%{$self});
    close CFG;
}

1;
[download]

#!/usr/bin/perl -w
# test1.pl

use strict;

use lib "$ENV{HOME}/perl";
use SimplestConfig;

my $cfg = SimplestConfig->new("$ENV{HOME}/test.cfg");
$cfg->{a} = 1;
$cfg->{b} = 2;
$cfg->{c} = 3;
[download]

#!/usr/bin/perl -w
# test2.pl

use strict;

use lib "$ENV{HOME}/perl";
use SimplestConfig;

my $cfg = SimplestConfig->new("$ENV{HOME}/test.cfg");
$cfg->{b} = 20;
$cfg->{d} = 4;
[download]

Test:

$ more test.cfg
test.cfg: No such file or directory
$ ./test1.pl
Can't read /home/laurent/test.cfg, creating it
$ more test.cfg
a=1
b=2
c=3
$ ./test2.pl
$ more test.cfg
a=1
b=20
c=3
d=4
[download]

Note it would discard any line not of the form key=value from an existing file.

And on another subject, Happy New Year to everybody.