use Sub::ScopeFinalizer qw( scope_finalizer );

sub local_sassign {
   my $r = \($_[0]);
   my $sentry = scope_finalizer { $$r = $_[0] } { args => [ $$r ] };
   $$r = $_[1];
   return $sentry;
}

sub f {
   my $sentry = local_sassign $SIG{ALRM}, \&alarm_handler;
   ...
}