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

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

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

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