You need to select the file handle that needs to be flushed before using $|=1.

sub flush {
   my $h = select($_[0]); my $af=$|; $|=1; $|=$af; select($h);
}

sub autoflush {
   my $h = select($_[0]); $|=$_[1]; select($h);
}

# Flush immediately without turning auto-flush on:
flush(*FILE);
flush(\*FILE);
flush($fh);

# Flush immediately and turn auto-flush on:
autoflush(*FILE,  1);
autoflush(\*FILE, 1);
autoflush($fh,    1);

# Turn auto-flush off:
autoflush(*FILE,  0);
autoflush(\*FILE, 0);
autoflush($fh,    0);
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Alternatively, if your file handle is an IO::Handle (or subclass) object, IO::Handle already provides methods:

# Flush immediately without turning auto-flush on:
$io->flush();

# Flush immediately and turn auto-flush on:
$io->autoflush(1);

# Turn auto-flush off:
$io->autoflush(0);
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