You need to select the file handle that needs to be flushed before using $|=1.

sub flush {
   my $h = select($_[0]); my $af=$|; $|=1; $|=$af; select($h);
}

sub autoflush {
   my $h = select($_[0]); $|=$_[1]; select($h);
}

# Flush immediately without turning auto-flush on:
flush(*FILE);
flush(\*FILE);
flush($fh);

# Flush immediately and turn auto-flush on:
autoflush(*FILE,  1);
autoflush(\*FILE, 1);
autoflush($fh,    1);

# Turn auto-flush off:
autoflush(*FILE,  0);
autoflush(\*FILE, 0);
autoflush($fh,    0);
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Alternatively, if your file handle is an IO::Handle (or subclass) object, IO::Handle already provides methods:

# Flush immediately without turning auto-flush on:
$io->flush();

# Flush immediately and turn auto-flush on:
$io->autoflush(1);

# Turn auto-flush off:
$io->autoflush(0);
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$| = 1, select $_ for select OUTPUT_HANDLE; before each critical print does the work... Ty

use IO::Handle;
STDOUT->autoflush(1);
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Thank you for the fast answers. I am sorry but the code is about 500 lines long. I can describe what it does. A special file will be downloaded, then I gather information from this file. With this information I connect to a server and post several ten thousad of times special coordinates to it in order to get special files back. These will be stored on hard disk and proccess further. From all these files another kind of information will be written to four different files as a short report. And here is the problem. These files are empty. I have tested the programm on a short file and it works fine. My problem is that I have to flush all this information on hard disk. I will try as suggested to select the filehandle that has to be flushed because I dont work with file:IO. Thank you again. If I have some more problems I will be back soon. :)