massa's code does not actually cause autovivification (perl 5.8.8) ...

use warnings; use strict;
use Data::Dumper;

my %p;
my %q = ( 1 => undef );

print Dumper( 'before', \%p );

my $r = grep $p{ $_ }, keys %q;

print Dumper( 'after', \%p );
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... I was also suspicious of autovivification but had to test that myself.