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%hash = (a   => 1,
         ab  => 1,
         abc => 1,
         ac  => 1);
$key = 'ab';

my @list_of_keys = sort grep /(^$search|$search$)/o, keys %hash;

my @list_of_keys = sort grep { substr($_, 0, length($search)) eq $sear
+ch || 
                               substr($_, -length($search)) eq $search
+ }, 
                        keys %hash;