It's not clear if you don't care about the file's content as long it has the right size, if so the code below will be quite quick.

my $req_len = shift || 100;

open FH, "+>", 'test.txt';
seek FH, $req_len - 1,0;
print FH 'X';
close FH;
[download]

or simpler ?

my $req_len = shift || 100;
truncate 'test.txt', $req_len;
[download]

HTH