use POSIX qw(:signal_h) ;

  my $sigset   = POSIX::SigSet->new ;
  my $blockset = POSIX::SigSet->new( SIGALRM ) ;

  sigprocmask(SIG_BLOCK, $blockset, $sigset );
  local $SIG{ALRM} = sub .... ;
  sigprocmask(SIG_SETMASK, $sigset );