for my $file (glob '*.mp4') {
    if ($file =~ /^[^0-9]*([0-9]+)[^0-9]*.mp4/) {
        rename $file, "[Books] $1.mp4" or die "$file: $!";
    } elsif ($file =~ /[0-9].*4/) {
        die "Several numbers found in $file.";
    } else {
        die "No number found in $file."
    }
}