No-one's mentioned using tie.

package Tie::Hash::OnlyIf;

sub STORE {
    my $self = shift;
    my ($key, $val) = @_;

    if ( $val ) {
        $self->{$key} = $val;
    }
}

package main;

tie my %data, Tie::Hash::OnlyIf;

$data{b} = $b;
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The only problem with this is that the casual reader will expect that the key 'b' will exist. But, it is a solution.

Oh - and tie's a lot slower than normal hashes, but performance is almost always overvalued. Going from 0.01 to 0.08 seconds per execution isn't noticeable.