#!/usr/bin/perl

use Fcntl;
use strict;

my $fname = nextunique( 'aaa', [ map { m/tmp_(\D+).txt$/ } glob('./tmp_*.txt') ] );
do {
    $fname = nextunique( $fname, [ map { m/tmp_(\D+).txt$/ } glob('./tmp_*.txt') ] );
} until open (FH, $fname, O_RDWR|O_CREAT|O_EXCL, 0666);
#   code follows
close FH;


sub nextunique {
    my ($test_item, $item_list) = @_;
    exit 1 unless UNIVERSAL::isa( $item_list, 'ARRAY' );

    foreach ( @{ $item_list } ) {
        if ( $_ eq $test_item ) {
            ++$test_item;
            last;
        }
    }
    return $test_item;
}