You are running a test and want to create a bunch of temporary files in a directory, or tables in a database with unique but not random names, and you want some minimal protection against race conditions. You might use something like nextuniq() for the files:

  
my $files_like = time;
my @existing   = map {m/tmp_(\d+)$/} glob('./tmp_*');

my $next_file  = nextuniq($files_like,\@existing);
open TEMP,">./tmp_$next_file" or die;
#    do something
close TEMP;
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or use it like this for the tables:

 my $tables_like  = 'test001';
 my $tables       =  [$dbh->tables()];

 my $test_table   = nextuniq($tables_like,$tables);
 my $sth = $dbh->prepare("CREATE TABLE $test_table (". 
              "RowID      INTEGER(3)  PRIMARY KEY,".
              "DrawingNo  VARCHAR(6)  NULL ,") or die $dbh->errstr;
 $sth->execute;              
#    do something
 $dbh->do("DROP TABLE $test_table");
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see rob_au's comments for making something like this more safe - the do{}until, is especially nice. (But, his version of the nextunique() procedure won't work the same and I see no advantage).
mkmcconn

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;
}
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