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;
    }
}
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