%x = (foo=>1,bar=>2,baz=>3); 

sub exists_srk(\%$){my($h,$k)=@_; map{$_ eq $k?$_:()}(sort reverse keys %$h)};

print exists_srk(%x,$_) for qw(foo bar baz bat)

##</code><code>##

sub exists_ggg (\%$) {
  my($h,$k) = @_;
  my $c;
  grep { len == len $k }
  grep /^\Q$k\E/ 
  grep { $c++ == 0 } %$h
};