I don't understand why this buffering occurs in this case as normally it doesn't

I'd say it's the other way around: buffering is the default behaviour. Only under certain circumstances, the behaviour is changed, for example when the handle is connected to a terminal (like STDOUT, typically). In this case, line-buffering (as opposed to block-buffering) is used, because an interactive mode of operation is assumed.

You could enable auto-flushing, though (on any handle):

use IO::Handle;

open my $fh, ">", "test.out" or die $!;
$fh->autoflush();

print $fh "...";  # is being written immediately
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(see also Re: Troubles with do...while loop and sleep (line- vs. block-buffered))