STDERR and STDOUT appear in the wrong order when piping to a file

a Perl script - test.pl:

print "OUT 1\n";
print STDERR "ERR 2\n";
print "OUT 3\n";
print STDERR "ERR 4\n";
print "OUT 5\n";

Run the script, piping all output into a file:

perl test.pl &> file.log

cat file.log

ERR 2

ERR 4

OUT 1

OUT 3

OUT 5

Output is in the wrong order :(

Run it again using unbuffer:

unbuffer perl test.pl &> file.log

cat file.log

OUT 1

ERR 2

OUT 3

ERR 4

OUT 5

Output is in the correct order :)

(source)

Another way:

script -c 'perl ~/temp/stream.pl' file.log

cat file.log

(source)

How are my Perl modules being loaded?

Large legacy system?
Confused about how your modules are being loaded?
Getting errors because they are loaded in the wrong order?

Try Devel::TraceUse