• print $fh q{}; is not needed. Turning autoflush on causes an immediate flush.

• $|++ is misleading. It implies $|-- will turn off autoflush, but it doesn't always. Be clear and use $| = 1 instead.

• local $|++; is a compile error. ("Can't modify postincrement (++) in local".)

• My tests shows that local $|; won't do what you think on block exit. It will set the autoflush flag for whichever file handle is selected when the blocks exits. That's the wrong handle.

Use this:

sub flush {
   my $osh = select($_[0]);
   my $oaf = $|;
   $| = 1;
   $| = $oaf;
   select($osh);
   return 1;
}
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Update: Added last two bullets and the code snippet.