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 $class.
    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 instance, 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 pass 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=$mod) to the method as a first param
print "my key: ".$mod->key()."\n";