$ cat file_gen.pl
#!/usr/bin/env perl
use strict;
use warnings;

open my $FH, '>', 'a_file.txt';
$|=1;
for (1 .. 1000) {
    print $FH "$_: here's 80 stars: ", "*"x80, "\n";
    my $t = time;
    print "$t: line: $_\n";
    sleep 1;
}