IMHO the best solution in this case where autovivification is wanted.

For completeness if the perl version is to old for defined-or, one could use a ref to the hash value.

$ perl
use Data::Dumper;

my $hash;

my $rval = \ $hash{a}{b}{c}{"hlst"};

$$rval = "" unless defined $$rval;

print Dumper \%hash;

__END__
$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => {
                                     'hlst' => ''
                                   }
                          }
                 }
        };
[download]

or you can define an defined-or-assign function to do so

use Data::Dumper;

my $hash;

defor( $hash{a}{b}{c}{"hlst"} , "");

print Dumper \%hash;

sub defor {
    $_[0] = $_[1] unless defined $_[0];
}

__END__
$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => {
                                     'hlst' => ''
                                   }
                          }
                 }
        };
[download]