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

my $files_like = 'aaa';

for (1..100){

   my $next_file = nextuniq($files_like,[map {m/tmp_(\D+)\.txt$/} glob('./tmp_*.txt')]);
    while (-e "./tmp_$next_file.txt"){
      $next_file = nextuniq($next_file,[map {m/tmp_(\D+)\.txt$/} glob('./tmp_*.txt')]);
      print "race condition\n";
    }  
    open TEMP,">./tmp_$next_file.txt" or die;
    #    do something
    close TEMP;
}

sub nextuniq{
  my ($test_item,$item_list) = @_;
  
  my $new_item;
  my $found = 1;
  if (! UNIVERSAL::isa( $item_list, "ARRAY" )){
    require Carp;
    Carp::croak( "Arg 2 must be an array reference\n"); 
    
  } 
  while ($found) {
    $found = 0;
    foreach $new_item (@$item_list) {
      if ($new_item eq $test_item) {
        $test_item++;
        $found = 1;
      }
    }
  }
  $new_item = $test_item;
  $new_item;
}