The module fails to create an object. As a result $overlay is undefined.

You should check the result of object creation and have a look at @Config::IniFiles::errors upon failure.

my $cfg = Config::IniFiles->new( -file => 'test.ini' ) 
  or die "Failed to create Config::IniFiles object: @Config::IniFiles:
+:errors\n";
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addendum: example:

0 linuxer@host /tmp $ cat /tmp/a.pl && perl /tmp/a.pl
#! /usr/bin/perl
use strict;
use warnings;

use Config::IniFiles;


my $overlay = Config::IniFiles->new(-file => 'test.ini')
        or die "Failure: @Config::IniFiles::errors\n";

$overlay->newval("section1", "arg1", "anotherval");
$overlay->WriteConfig(-delta=>1);

__END__

Failure: Empty file treated as error
255 linuxer@host /tmp $
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