Am I the only one to use $| = 1 with mod_perl?

You might be. Is it ever necessary? You said you want to be as lean as possible, but autoflush ($|) makes scripts slower.

Or, imagine a common CPAN module would start to set $| nonzero in a new verison...

That's why one should always use local when setting special variables. Imagine what would happen if one changes $/ without localizing it first. Even not localizing $_ can be problematic. Under mod_perl you should always use local with special variables. (Don't local $ENV{key} as this is buggy: localize the entire hash)

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

Actually, regarding my problem, it doesn't make a difference to localize $|.

My server now looks like this, but has the same hang

#!/usr/bin/perl -w

use strict;

    local $| = 1;

    print "Content-Encoding: iso-8859-1\015\012",
                "Content-Type: text/plain\015\012",
                "\015\012";

    warn "DEBUG: the content length is $ENV{CONTENT_LENGTH}\n";

    my $returncode = read(STDIN, my $post_content, $ENV{CONTENT_LENGTH
+});

    warn "DEBUG: the read returncode was $returncode\n";
    warn "DEBUG: Finished.";
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Stas Bekman has summed up why one would use $| = 1 under mod_perl quite well in mod_perl: Performance Tuning.