This is another approach. Read the file names in the output directory, filter out the ones that aren't really simple files (maybe there is some directory with a similar name), get the revision numbers of those files, get the max revision number and then add one to that.

#!/usr/bin/perl -w
use strict;
use List::Util qw(max);  # List::Util is a core module
                         # that means you don't have to
                         # install it - its already there.

my $output_dir = 'C:/TEMP';
my $program_root_name = "some_prog";

opendir DIR, $output_dir 
    or die "cannot open $output_dir for reading $!";

my @current_numbers =                           
  map { /^$program_root_name(\d+)$/ ? $1 : () } # just the numbers
  grep{ -f "$output_dir/$_"}readdir DIR;       # only "real" files
                                               # not directories
closedir DIR;  # purely optional                                

foreach (@current_numbers)  #intermediate print routine...
{
   print "$output_dir/$program_root_name$_ exists!\n";
}

my $max_cur_number = max(@current_numbers);
my $new_max = ++$max_cur_number;
print "The next highest number is $new_max\n";
print "Full name of next file in sequence would be: ".
      "$output_dir/$program_root_name$new_max\n";
     

__END__
The above prints:
Note: I made the dummy files: some_prog1 and some_prog3...
C:/TEMP/some_prog1 exists!
C:/TEMP/some_prog3 exists!
The next highest number is 4
Full name of next file in sequence would be: C:/TEMP/some_prog4
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