#!perl -pi~
my $p;
my $done_replacement = 0;
while (<>) {
    if (/^A (.*) B$/) {
        $p = $1;
    }
    elsif (defined $p && !$done-replacement) {
        s/^(C .*)$p$/$1 Q/;
        $done_replacement = 1;
    }
}