sub get_defaults {
    return +{
      output_type => 'excel',
      output_dir  => '/tmp',
      ...
    };
};

##</code><code>##

use YAML 'LoadFile';

sub config_from_file {
    my( $filename ) = @_;
    LoadFile( $filename )
}

##</code><code>##

sub config_from_ENV {
    my ( $env ) = @_;
    {
        output_type => $env->{MYAPP_output_type},
        ...
    }
}

##</code><code>##

sub config_from_ARGV {
    my( $argv ) = @_;
    GetOptionsFromArray (
        'f|config_file:s' => \my $config_file,
        't|output_type:s' => \my $output_type,
    );
    return +{
        config_file => $config_file,
        output_type => $output_type,
    },

##</code><code>##

my $defaults = get_defaults();

my $argv = config_from_ARGV( \@ARGV );
my $config_file = $argv->{config_file} || 'myconfig.yml';
my $file = config_from_file( $config_file );
my $env  = config_from_ENV( \%ENV );

# Merge the things
my $final_config = $defaults;
for my $cfg ($config_file, $env, $argv) {
    for my $entry (keys %$cfg) {
        if(defined $cfg->{$entry}) {
            $final_config->{ $entry } = $cfg->{ $entry };
        };
    };
};