open my $out, '>', 'bigfile' or die $!;
my $l = 0;
while( $l < 350*1024*1024 ){
    my $part1 = join '', map { ('A'..'Z','a'..'z',0..9)[rand(62)] } (0..(rand(30)+10));
    my $part2 = join '', map { ('A'..'Z','a'..'z',0..9)[rand(62)] } (0..(rand(30)+10));
    my $output = "$part1*$part2\n";
    $l += length $output;
    print $out $output;
}

my $filesize = -s 'bigfile';
say 'File size: ', $filesize;