it's not faster

It *is* faster than your method.

Not as fast as writing 10MB of a single character for sure, but then you are calling a function, rand 10 million times; it has to be slower.

The only question is; how much faster do you need it to be?

OK, I'm confused. I wanted to see how much faster BrowserUk's version was. But Benchmark is showing that it is slower.

#!/usr/bin/perl

use warnings;
use strict;
use Benchmark qw( cmpthese );

my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );

open my $fh, ">", "/dev/null";

cmpthese( -20, {
            Original => sub { generateRandomFile($fh, 5) },
            BrowserUk => sub {genFile($fh, 5) },
        });


sub generateRandomFile
{
    my( $gen_file_h, $final_size ) = @_;

    for (my $mbytes = 0; $mbytes < $final_size; $mbytes ++) 
        {
            my $string="";
            my @chars = split(" ","a b c d e f g h i j k l m n o p q r
+ s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
+- _ ! & ? = 0 1 2 3 4 5 6 7 8 9");
            for (1..1048576)
                {
                    my $rand = int(rand(68));
                    $string .= $chars[$rand];
                }
            print $gen_file_h $string;
        }
}
sub genFile {
    my( $fh, $mb ) = @_;
    my $buf = chr(0) x 1024**2;
    for my $chi ( 1 .. $mb ) {
        substr $buf, $_, 1, $chars[ rand @chars ] for 0 .. 1024**2-1;
        print $fh $buf;
    }
}
__END__

          s/iter BrowserUk  Original
BrowserUk   1.34        --      -15%
Original    1.13       18%        --
[download]

OK, I'm confused.

You're not confused; you are correct!

I don't know what I did different late last night, but retrieving the same command lines from my console history I used to test this yesterday, that showed my way taking almost half the time, today show it taking ~15% longer.

So, my apologies to the OP and thank you Mr Anonymonk for checking me.

By way of recompense I offer this version which takes 1/4 of the time, at the cost of being very slightly less fair:

my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );
sub genFile {
    my( $fh, $mb ) = @_;
    my @buf = ( @chars ) x 61; $#buf = 4096;
    for( 1 .. $mb * 256 ) {
        print $fh join'', shuffle @buf;
    }
}
[download]

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.