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."
    }
}

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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]

In reply to Re^3: Removing characters in a file name by choroba
in thread Removing characters in a file name by andy2020

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