You mentioned portability was a requirement, so you should use File::Spec to build paths. "/" is not the file seperator on Macs, for example.

I have a script that needs to determine the oldest file in a particular directory

If all you're concerned about is which file is the oldest, there's no need to sort:

sub get_oldest {
   my ($dir) = @_;
   my $oldest;
   my $oldest_time;
   my $file_spec = File::Spec->catfile($dir, '*.pl');
   foreach (glob $file_spec") {
      my $time = (stat $_)[10];
      if (!$oldest_time || $time < $oldest_time) {
         $oldest      = $_;
         $oldest_time = $time;
      }
   }
   return $oldest;
}
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I don't know how efficient glob is. You can get rid of it:

use DirHandle  ();
use File::Spec ();

sub get_oldest {
   my ($dir) = @_;
   my $oldest;
   my $oldest_time;
   my $dh = DirHandle->new($dir);
   while (defined($_ = $dh->read())) {
      next unless (/\.pl$/i);
      my $full_path = File::Spec->catfile($dir, $_);
      my $time = (stat $full_path)[10];
      if (!$oldest_time || $time < $oldest_time) {
         $oldest      = $_;
         $oldest_time = $time;
      }
   }
   return $oldest;
}
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