So my question is, how can I check when I open a process, if the command really exists?

Just try to run the command and see if that fails. Don't fail fatally (die()) if you do not want to.

use strict;
use warnings;

my $run_cmd = "dira";

if ( open(my $CMD, "$run_cmd |") ) {
  while( <$CMD> ) {
      print "$_";
  }
  close($CMD);
} else {
  die "Problem executing command $run_cmd - $!";
  # ... or something non-fatal
}
[download]

Alternatively, you could wrap your stuff into an eval { ... } block and then examine $@ (e.g. Problem executing command in the example above). You could also try one of the many CPAN modules that supports handling of exceptions.

Update:

Another possibility would be to loop through all pathes in the environment variable PATH and then checking if the command exists.

There is this ancient thread and some CPAN modules like Dir::Which or File::Which that can be used to perform apriori checks - but see cdarke's comments below.

Update2:
The program above was tested and worked fine under Linux. Since I am not a Windows-expert, I can only do wild guesses on what the problem could be:

• cmd is started successfully but then fails to run dira?
• open() might return something that Perl thinks is true (e.g. -1)?
• dira exists somewhere, but prints an error...?
• copy/paste error?
• ...