#!/usr/bin/perl -Wl
use strict;
use File::Spec qw| catdir tmpdir |;

my $tmpdir;

sub repl {
    my @CHARS = ("a".."z", "0".."9");
    local $_ = pop;
    s/ X / $CHARS[int rand $#CHARS] /gex;
    $_;
}

do {
    $tmpdir = File::Spec->catdir( File::Spec->tmpdir , repl ("asd-XXXX.mbox") );
} until (!-e $tmpdir);

print $tmpdir;