my $f;

$f = sub {
    if (/alpha/) {
        do_alpha();
        $f = \&do_beta;
    } else {
        do_beta();
    }
};

while (<DATA>) {
    $f->();
}


sub do_alpha {
    print "do_alpha(): \$f = $f, \$_ = $_";
}
sub do_beta {
    print "do_beta():  \$f = $f, \$_ = $_";
}


__DATA__
beta
beta
beta
beta
alpha
beta
beta
beta
beta
beta
beta
beta
beta

##</code><code>##

do_beta():  $f = CODE(0x796e88), $_ = beta
do_beta():  $f = CODE(0x796e88), $_ = beta
do_beta():  $f = CODE(0x796e88), $_ = beta
do_beta():  $f = CODE(0x796e88), $_ = beta
do_alpha(): $f = CODE(0x796e88), $_ = alpha
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta
do_beta():  $f = CODE(0x7c6ee8), $_ = beta