my %line;
{
    open my $fhA, '<', 'A' or die $!;
    while (<$fhA>) {
        $line{$.} = undef if /$X/;
    }
}

##</code><code>##

use Fcntl qw/:flock/;
{
    my @file;
    open my $fhB, '+<', 'B' or die $!;
    flock $fhB, LOCK_EX or die $!;
    while (<$fhB>) {
        s/$Y/$Z/ if exists $line{$.};
        push @file, $_;
    }

    truncate $fhB, 0 or die $!; # set up to overwrite file
    print $fhB @file;
}

##</code><code>##

use Tie::File;
tie my @file, 'Tie::File', 'B' or die $!;

{
    open my $fhA, '<', 'A' or die $!;
    while (<$fhA>) {
        $file[$. - 1] =~ s/$Y/$Z/ if /$X/;
    }
}