use Fcntl qw(O_CREAT O_EXCL O_WRONLY);
my $i;
while(1) {
    my $dest = compose_name($base, $ext, $i++); 
    if(sysopen my $fh, $dest, O_CREAT | O_EXCL | O_WRONLY) {
        close $fh;
        copy $source, $dest;
        last;
    }
}