I'm sorry for not being clear before. There are hundreds of files in the directory and not just one. All names are consistent in naming such as below

Jane Doe & John Doe - 124 - How to Invest.mp4
Jane Smith & John Smith - 100 - Stocks.mp4
Jane Black & John Black - 90 - Bonds.mp4
[download]

Here's what the end results should be

[Books] 124.mp4
[Books] 100.mp4
[Books] 90.mp4
[download]

Thanks

Loop over the files, you can use glob to get the list. Make sure there is a number to rename to:

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."
    }
}
[download]

You can replace the second and third die with a warn if you just want to skip the files that weren't renamed.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Thanks for the reply. It's working now. Unfortunately I found out that I have some files that have the following format too

JohnDoe-JaneDoe-399-1080p.mp4

So the only common thing of this new file format with the previous format is the number between the two dashes. There could be spaces before or after the two dashes too. So the two formats that I have will be below

JohnDoe-JaneDoe-399-1080p.mp4
John Doe & Jane Doe - 458 - Bonds.mp4
[download]

As you can see, there could be spaces before and after the dashes so the only consistent thing between the two file naming formats is there will be a number between two dashes

Is there a way to modify the if statement to accommodate both file formats?

I like the idea from choroba, but I would perhaps do it a bit differently. The OP doesn't specify what to do if the rename cannot happen. I would just "die" in that case with an error message. I would only process .mp4 files that match the pattern, but again that is up to the OP to decide how to handle an .mp4 file that doesn't match the pattern.

use strict;
use warnings;

foreach my $file ('Jane Doe & John Doe - 124 - How to Invest.mp4',
                  'Jane Smith & John Smith - 100 - Stocks.mp4',
                  'Jane Black & John Black - 90 - Bonds.mp4',
                  'Jane Black & John Black - 120 - Bonds2Invest.mp4',
                  'Jane Black & John Black - 123 - Bonds 2 Invest.mp4'
+,
                  )
{
   if ( (my $num) = $file =~ / (\d+) - [\w ]+.mp4$/ )
   {
      #rename $file, "[Books] $num.mp4" or die "Cannot rename $file to
+ $num.mp4 - probable cause: duplicate number: $!";
      print "$num.mp4\n";
   }  
}
__END__
124.mp4
100.mp4
90.mp4
120.mp4
123.mp4
[download]