You can read the parent directory:

my $dir = "whatever";
local *DIR;
my $file;
my $newest;
my $path;

opendir(DIR, $dir) or die "Failed to open $dir: $!\n";

while (defined($file = readdir(DIR)))
{
    $path = "$dir/$file";

    if ( (-d $path) && ($file =~ /^PC/) )
    {
        $newest = $path
          if ( !defined($newest) || 
               ((stat($path))[9] > (stat($newest))[9]) );
    }
}

closedir(DIR);
chdir($newest);
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This may be easier using File::Find and I'd probably turn it into a callable sub in either case.