qq looks more suitable in this case (for OP: it justs double-quotes the parameter and returns a scalar)

I've been looking at the source for the pm and wonder if I shouldn't imitate it in significant ways. source listing for Translate.pm For example, should I use Readonly for these values?

my ( $REST_HOST, $REST_URL, $CONSOLE_URL, %SIZE_LIMIT_FOR );
{
    Readonly $REST_HOST      => 'translation.googleapis.com';
    Readonly $REST_URL       => "https://$REST_HOST/language/translate
+/v2";
    Readonly $CONSOLE_URL    => "https://console.developers.google.com
+/cloud-resource-manager";
    Readonly %SIZE_LIMIT_FOR => (
        translate => 2000,    # google states 2K but observed results 
+vary
        detect    => 2000,
        languages => 9999,    # N/A
    );
}
[download]

Also, I want to build a central hash to store the data. Values would go in with Getopt::Long and end up in %self . This treatment creates a class of it. One thing I don't see is where the value of $class gets passed to the sub new:

sub new {
    my ( $class, $param_hr ) = @_;
 
    my %self = (
        key            => 0,
        format         => 0,
        model          => 0,
        prettyprint    => 0,
        default_source => 0,
        default_target => 0,
        data_format    => 'perl',
        timeout        => 60,
        force_post     => 0,
        rest_url       => $REST_URL,
        agent          => ( sprintf '%s/%s', __PACKAGE__, $VERSION ),
        cache_file     => 0,
        headers        => {},
    );
[download]

Edit: 2 problems with this: 1) use My::Module; breaks things when the package is actually implemented in same file, so comment that out. 2) wrong order ot bless. The correct is bless $self, $class;, I use bless $self => $class; to be reminded of the correct order.

package My::Module;
sub new {
    my ( $class, $param_hr ) = @_;
    $param_hr = {} unless defined $param_hr;
    my $self = { # hashref or arrayref
        key            => 0,
        format         => 0,
        #...
    };
    bless $self => $class; # now your hash is an object of class $clas
+s.
    if( exists $param_hr->{'key'} ){ $self->key($param_hr->{'key'}) }
    else { warn "param 'key' is required."; return undef }
    return $self; # return the hash, sorry now blessed into a class in
+stance, hallelujah
}
# get or set the key
sub key {
  my $self = $_[0];
  my $akey = $_[1]; # optional key
  print "you called key() method on object '$self'\n";
  if( defined $akey ){
    print "key() : changing key to '$akey'\n";
    $self->{'key'} = $akey;
  }
  return $self->{'key'};
}
1;
package main;
# uncomment only if My::Module is in separate file:
#use My::Module;

# the notation module->new adds "module" as the first argument to the 
+new() params.
# so here is how $class is set in your question, although YOU dont pas
+s class, Perl does.
my $mod = My::Module->new({
'key' => 123,
});
die unless defined $mod;
# the same notation applies when calling the method
# Perl passes the object ref ($self=$mod) to the method as a first par
+am
print "my key: ".$mod->key()."\n";
[download]

Sometimes defining constants via Readonly or otherwise is useful