eval
{
     local $SIG{ALRM} = sub { die "alarm\n" };
     alarm 20;
     $CHILD = fork();
     if( $CHILD == 0 )
     {
          exec( "my external command" );
     }
     waitpid( $CHILD, 0 );
     alarm 0;
};
if( $@ )
{
     die unless $@ eq "alarm\n";
     kill 9, $CHILD
}