Just use File::Copy (more efficient than reading/writing one line at a time):

use File::Temp qw/tempfile/;
use File::Copy;

my ($fh, $fname) = tempfile();
syswrite $fh, "FIRST LINE\n";
copy($file1, $fh) or die "Error copying: $!";
close $fh;
[download]

rename $fname, $file1;