Ok, here's a solution that does it on a single pass regardless of order, it takes advantage of the fact that the array elements are references to the anonymous hashes (perlreftut, perlref, perldsc):

my ($source,$target);
for my $item (@$all) {
    if ( $item->{code} eq 'XXY' ) {
        $source = $item;
        $target->{expdate} = $item->{expdate} if defined $target;
    } 
    if ( $item->{code} eq 'YYK' ) {
        $target = $item;
        $item->{expdate} = $source->{expdate} if defined $source;
    }
}
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