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

##</code><code>##

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

##</code><code>##

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