No, the key will only autivify when you assign a value to it.

Prove:

#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

my %h;

print $h{key} ? $h{key} : "nokey";

print "\n", Dumper (\%h);
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See? The hash stays emtpy.

Update: exists, defined and the check for truth do different things. Consider:

#!/usr/bin/perl
use strict;
use warnings;

my %h = ( a => "", b => undef, c => "true");

for ( qw(a b c d) )
{
    print "key: $_ is " . ($h{$_}         ? "" : "not ") . "true\n";
    print "key: $_ is " . (defined $h{$_} ? "" : "not ") . "defined\n"
+;
    print "key: $_ is " . (exists  $h{$_} ? "" : "not ") . "existing\n
+";
}

#key: a is not true
#key: a is defined
#key: a is existing
#key: b is not true
#key: b is not defined
#key: b is existing
#key: c is true
#key: c is defined
#key: c is existing
#key: d is not true
#key: d is not defined
#key: d is not existing
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