DB<21> $par{"margin-left"} = "a" 

  DB<22> ($actual) = grep { $par{$_} ne "a" } qw/margin-left margin-horizontal margin/

  DB<23> p $actual
margin-horizontal

##</code><code>##

$chain{margin}    = [qw/margin-left margin-horizontal margin/ ];
# etc
($actual{$head}) = grep { $par{$_} ne "a" } @{ $chain{$head} };