I guess you got a "Name "main::OUTPUT_HANDLE" used only once: possible typo" warning with the code you posted

Try

my $old_fh = select(STDOUT);
$| = 1;
select($old_fh);
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• The first select changes the default output filehandle to STDOUT and returns the current default output filehandle to be saved in $old_fh.
• $|=1 turns on autoflushing on the current output filehandle
• select($old_fh) restores the original output filehandle

Generally, if you just want to turn on autoflushing on STDOUT, a simple $|=1 will do.