Two solutions : 1) print to another file and move it to the first after you close it

2) put the whole content in a @buffer, and save it to the file after you finish.

In your case I would prefer the first. The second one is better when you want to process the contents another time before printing/saving

Just as an example I will show a snippet of a backup script I have. The first part collects the output messages of a program hilighting the interesting lines
. The second is the function that sends that as an email.

{...}
    foreach(@fsystem){
        $fs=$_;
        print "Backup do $mountpoint{$fs} ($fs)\n";
        open (FS,"/sbin/dump $param \/dev/$mountpoint{$fs} 2>&1 |");
        push(@messages,("\nBackup de /dev/$mountpoint{$fs} ($fs)\n\n\n
+"));
        while($linha=<FS>){
            push(@messages,$linha);
            if ($linha=~ /DUMP IS DONE/){ 
                print color 'green';
            }
           if ($linha=~/error/)&&($linha=~/ERROR/)&&($linha=~/Error/){
               print color 'red';
           }
            print $linha;
            print color 'reset';
        }
        close(FS);
    }
{...}
sub EnviaMail(){

    $smtp = Net::SMTP->new('256.256.256.256');
    $smtp->mail($sender);
    $smtp->to($resp);
    $smtp->data();
    $smtp->datasend("To: $resp\n");
    $smtp->datasend("From: \"Backup $localhost\" \<$sender>\n");
    $smtp->datasend("Subject: Backup $localhost\n");
    $smtp->datasend("\n");

    foreach $linha (@messages) {
        $smtp->datasend($linha);
    }

    $smtp->dataend();

    $smtp->quit;
}
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Zenn