If I understand your question correctly, line 106 is what you're looking for.

  DB<105> $href = { KEY1 => { KEY11 => "c", KEY12 => "b" } }
 => { KEY1 => { KEY11 => "c", KEY12 => "b" } }
 
  DB<106>  @{$href->{KEY1}}{qw/KEY11 KEY12/}=("c" ,"d");
 => ("c", "d")
 
  DB<107> $href
 => { KEY1 => { KEY11 => "c", KEY12 => "d" } }
[download]

you are getting an anonymous array because you enclosed your slice-range in brackets.