These command loops are tricky to write.

I should say that I do not often write menu driven command line interfaces because they are hard to automate. The UI and process is just easier with a Unix style command.

Perhaps here ">cold comp5" would be better than a menu choice?

Anyway, first off, I do have a decided preference for while loops as opposed to do{...}until loops. The reason is that I like to see the loop ending condition at the front of the loop. But there is no real issue with using a do loop.

I guess the general "rules" are:
1) prompt the user in a clear way.
2) ignore completely any leading or trailing spaces.
3) a blank line is just a re-prompt (not an error).
4) protect the user from "faults" - if 0 is not allowed then that is an error input - error message and reprompt.
5) With a menu, have a clear way for the user to "quit" without going to the extreme of the CTRL-C.
6) Quality of error messages vary widely - application specific. There can be a lot of time spent on this. How much depends
upon who your users are. Same thing for help messages, etc.

Here is another way to write this menu loop:

#!/usr/bin/perl -w
use strict;

my $computer;
my @pcs = qw(comp1 comp2 comp3 comp4 comp5);

while ( $computer = prompt4number(@pcs),
        $computer !~/^\s*(Q|quit|exit)\s*$/i)
{
      next if $computer =~ /^\s$/; #simple re-prompt on blank lines
      
      if( $computer !~ /^\s*\d\s*$/ or $computer>@pcs or
          $computer == 0)
      {
         print "Illegal Entry -- try again!\n\n";
         next;
      }
      
      print "OK - cold starting $pcs[$computer-1]\n";
      exit(0); 
}

print "Program would just exit ...\n";

sub prompt4number
{
   my @pc = @_;
   print "\nWhich computer would you like to cold start?\n";
   
   my $menu =1;
   print $menu++, " - $_\n" foreach @pc; 
   print "enter Q|q|quit|exit to stop this program!\n";
   return <STDIN>;
}   
   
__END__
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