I want to run a series of Unix commands -- killing them if they take too long -- and timing how long each command takes to run. Below is my first attempt. It is very easy to have subtle bugs in these sorts of programs, so I would appreciate any advice on how to improve the code below.

use strict;
use warnings;
use POSIX ":sys_wait_h";
use Time::HiRes qw(time sleep);
use Benchmark;
# use Benchmark ':hireswallclock';    # 5.8 and above

select(STDERR);$|=1;select(STDOUT);$|=1;   # autoflush

sub slurp_file {
   my $file = shift; local $/;
   open(my $fh, $file) or die "error:open '$file': $!";
   <$fh>;
}

my $Pid;
my $Outf = "out-$$.tmp";
my $Errf = "err-$$.tmp";

sub write_result {
   my ($killsig, $elap, $user, $sys) = @_;
   my $rc  = $? >> 8;      # return code of command
   my $sig = $? & 127;     # signal it was killed with
   warn "pid=$Pid, rc=$rc sig=$sig (killsig=$killsig)\n";
   warn "  elapsed=$elap user=$user sys=$sys\n" if defined($elap);
   my $outstr = slurp_file($Outf);
   my $errstr = slurp_file($Errf);
   unlink($Outf) or die "error: unlink '$Outf': $!";
   unlink($Errf) or die "error: unlink '$Errf': $!";
   warn "cmd stdout='$outstr'\n";
   warn "cmd stderr='$errstr'\n";
}

sub run_cmd {
   my $cmd = shift;
   warn "\nrun_cmd '$cmd' at " . scalar(localtime) . "\n";
   my $b0 = Benchmark->new();
   my $t0 = time();
   defined($Pid = fork()) or die "error: fork: $!";
   if ($Pid == 0) {
      ### child
      open(STDOUT, '>'.$Outf) or die "error create '$Outf': $!";
      open(STDERR, '>'.$Errf) or die "error create '$Errf': $!";
      exec($cmd);
      # my @args = split(' ', $cmd); exec { $args[0] } @args;
      die "error: exec: $!";
   }
   ### parent
   warn "in run_cmd, waiting for pid=$Pid\n";
   waitpid($Pid, 0);
   my $t1 = time();
   my $b1 = Benchmark->new();
   my $bd = timediff($b1, $b0);
   my ($real, $child_user, $child_sys) = @$bd[0,3,4];
   write_result(0, $t1 - $t0, $child_user, $child_sys);
}

# Run command $cmd, timing out after $timeout seconds.
# See Perl Cookbook 2nd edition, Recipe 16.21
# See also perlfaq8 "How do I timeout a slow event".
# Return 1 if $cmd run ok, 0 if timed out.
sub run_for {
   my ($cmd, $timeout) = @_;
   my $diestr = 0;
   eval {
      local $SIG{ALRM} = sub { die "alarm clock restart" };
      alarm($timeout);    # schedule alarm in $timeout seconds
      eval { run_cmd($cmd) };
      $diestr = $@ if $@;
      alarm(0);           # cancel the alarm
   };
   $diestr = $@ if $@;
   alarm(0);              # race condition protection
   return 1 unless $diestr;
   return 0 if $diestr =~ /alarm clock restart/;
   die;
}

sub kill_it
{
   warn "kill_it: pid=$Pid\n";
   kill(0, $Pid) or warn("pid $Pid is not alive\n"), return;
   my $waitpid; my $killsig = 15;
   kill($killsig, $Pid); sleep(0.1);
   for (1..3) {
      $waitpid = waitpid($Pid, &WNOHANG);
      last if $waitpid == $Pid;
      sleep(1);
   }
   if ($waitpid != $Pid && kill(0, $Pid)) {
      $killsig = 9;
      warn "pid $Pid not responding, resorting to kill 9\n";
      kill($killsig, $Pid);
      waitpid($Pid, 0);
   }
   write_result($killsig);
}

my @cmds = (
   'ls -l',
   'sleep 15',
   'sleep 4',
   'echo hello-stdout; echo hello-stderr >&2',
);
for my $cmd (@cmds) { run_for($cmd, 10) or kill_it() }
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Update: see also Timing Windows commands.