$hash{$_} ||= 1 + keys %hash for @list;
$hash{$_} = $hash{$_} || 1 + keys %hash for @list;

##</code><code>##

use Data::Dump::Streamer;
my (%hash,%mash);
@list=('A'..'C');
$hash{$_} ||= 1 + keys %hash for @list;
$mash{$_} = $mash{$_} || 1 + keys %mash for @list;
Dump(\%hash,\%mash)->Names('*hash','*mash')->Out();
__END__
%hash = (
          A => 2,
          B => 3,
          C => 4
        );
%mash = (
          A => 1,
          B => 2,
          C => 3
        );