A very good tip. I thought I was defending against, for example, passing in a hashref by mistake. But it appears that perl will throw an error in this case. Yay perl!

However, I had to modify the sub you proposed to get it to work:

 return join '', map { "( ( $_ ) & );" } @{ $_[0] };

Here's the code with a hashref, which gets thrown a nice error.

#!/usr/bin/perl
use strict;
use warnings;
use Carp qw(confess);

my @commands = map {
  "echo $_; sleep 2";
} qw(a b c d e);

#does first, waits two seconds, does the second, waits two seconds, et
+c. (should take about ten seconds)
#for my $bash_command ( @commands  ) {
#  time_bash_command($bash_command);
#}

#does commands in parallel. (should take about two seconds)
my $parallel_running_command = parallelize_bash_commands([@commands]);
$parallel_running_command = parallelize_bash_commands( { a => 'sleep 2
+', b=> 'sleep 2', c => 'sleep 2'  } );


sub parallelize_bash_commands {

   return join '', map { "( ( $_ ) & );" } @{ $_[0] };
   #return join '', map { "( ( $_ ) & );" } @{ my $input = shift or di
+e "no input" }; # alternative -- also works, and checks if you forgot
+ to pass in a var.
   time_bash_command($parallel_running_command);
}

sub time_bash_command {
  my $command = shift or die "no command";
  $command = "time `$command`";
  print "$command\n";
  print `$command`;
}
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