Readonly::Hash $myHash => ("key1" = "val1",); 
$myTempValue = $myHash{"this_key_does_not_exist"};
##</code><code>##

if ($myHash{"unknown_if_this_key_exists_yet_or_not"}){
    print "values in list are: @{$myHash{\"unknown_if_this_key_exists_yet_or_not\"}}\n";
}
##</code><code>##

if (exists($myHash{"unknown_if_this_key_exists_yet_or_not"}) and $myHash{"unknown_if_this_key_exists_yet_or_not"}){
    print "values in list are: @{$myHash{\"unknown_if_this_key_exists_yet_or_not\"}}\n";
}
##</code><code>##

use strict; 
no autovivification;    #<-- include in all programs, all the time!

my %myHash =();

print "values in list are: @{$myHash{\"unknown_if_this_key_exists_yet_or_not\"} || []}\n";